diff --git a/frontend/src/lib/components/TranscriptionEditView.svelte b/frontend/src/lib/components/TranscriptionEditView.svelte
index 85becbc2..c156f6c0 100644
--- a/frontend/src/lib/components/TranscriptionEditView.svelte
+++ b/frontend/src/lib/components/TranscriptionEditView.svelte
@@ -73,7 +73,7 @@ $effect(() => {
 
 $effect(() => {
 	function onBeforeUnload() {
-		autoSave.flushViaBeacon();
+		autoSave.flushOnUnload();
 	}
 	window.addEventListener('beforeunload', onBeforeUnload);
 	return () => {
diff --git a/frontend/src/lib/hooks/__tests__/useBlockAutoSave.svelte.test.ts b/frontend/src/lib/hooks/__tests__/useBlockAutoSave.svelte.test.ts
index dd13a7a6..739fb432 100644
--- a/frontend/src/lib/hooks/__tests__/useBlockAutoSave.svelte.test.ts
+++ b/frontend/src/lib/hooks/__tests__/useBlockAutoSave.svelte.test.ts
@@ -1,4 +1,4 @@
-import { describe, it, expect, vi, beforeEach, afterEach } from 'vitest';
+import { describe, it, expect, vi, beforeEach, afterEach, type Mock } from 'vitest';
 
 const mockSaveFn = vi.fn<(blockId: string, text: string) => Promise<void>>();
 
@@ -82,3 +82,82 @@ describe('createBlockAutoSave', () => {
 		expect(mockSaveFn).not.toHaveBeenCalled();
 	});
 });
+
+describe('flushOnUnload', () => {
+	let mockFetch: Mock;
+
+	beforeEach(() => {
+		vi.useFakeTimers();
+		mockSaveFn.mockClear();
+		mockSaveFn.mockResolvedValue(undefined);
+		mockFetch = vi.fn().mockResolvedValue(new Response(null, { status: 200 }));
+		vi.stubGlobal('fetch', mockFetch);
+	});
+
+	afterEach(() => {
+		vi.useRealTimers();
+		vi.unstubAllGlobals();
+	});
+
+	it('sends a PUT request with keepalive:true for each pending block', () => {
+		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
+		as.handleTextChange('block-1', 'hello');
+		as.handleTextChange('block-2', 'world');
+		as.flushOnUnload();
+
+		expect(mockFetch).toHaveBeenCalledTimes(2);
+		expect(mockFetch).toHaveBeenCalledWith(
+			'/api/documents/doc-1/transcription-blocks/block-1',
+			expect.objectContaining({
+				method: 'PUT',
+				keepalive: true,
+				body: JSON.stringify({ text: 'hello' })
+			})
+		);
+		expect(mockFetch).toHaveBeenCalledWith(
+			'/api/documents/doc-1/transcription-blocks/block-2',
+			expect.objectContaining({
+				method: 'PUT',
+				keepalive: true,
+				body: JSON.stringify({ text: 'world' })
+			})
+		);
+	});
+
+	it('does not call navigator.sendBeacon', () => {
+		const sendBeaconSpy = vi.spyOn(navigator, 'sendBeacon').mockReturnValue(true);
+		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
+		as.handleTextChange('block-1', 'text');
+		as.flushOnUnload();
+
+		expect(sendBeaconSpy).not.toHaveBeenCalled();
+	});
+
+	it('does nothing when there are no pending edits', () => {
+		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
+		as.flushOnUnload();
+
+		expect(mockFetch).not.toHaveBeenCalled();
+	});
+
+	it('cancels the debounce timer so saveFn is not also called', async () => {
+		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
+		as.handleTextChange('block-1', 'text');
+		as.flushOnUnload();
+
+		await vi.advanceTimersByTimeAsync(2000);
+		expect(mockSaveFn).not.toHaveBeenCalled();
+	});
+
+	it('does not send fetch if debounce already fired and pendingTexts is empty', async () => {
+		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
+		as.handleTextChange('block-1', 'text');
+		await vi.advanceTimersByTimeAsync(1500);
+		// debounce has fired; pendingTexts should be empty now
+		mockFetch.mockClear();
+
+		as.flushOnUnload();
+
+		expect(mockFetch).not.toHaveBeenCalled();
+	});
+});
diff --git a/frontend/src/lib/hooks/useBlockAutoSave.svelte.ts b/frontend/src/lib/hooks/useBlockAutoSave.svelte.ts
index a627f50e..07dc5692 100644
--- a/frontend/src/lib/hooks/useBlockAutoSave.svelte.ts
+++ b/frontend/src/lib/hooks/useBlockAutoSave.svelte.ts
@@ -94,12 +94,15 @@ export function createBlockAutoSave({ saveFn, documentId }: Options) {
 		saveStates.delete(blockId);
 	}
 
-	function flushViaBeacon(): void {
+	function flushOnUnload(): void {
 		for (const [blockId, text] of pendingTexts) {
 			clearDebounce(blockId);
-			const url = `/api/documents/${documentId}/transcription-blocks/${blockId}`;
-			const body = JSON.stringify({ text });
-			navigator.sendBeacon(url, new Blob([body], { type: 'application/json' }));
+			void fetch(`/api/documents/${documentId}/transcription-blocks/${blockId}`, {
+				method: 'PUT',
+				headers: { 'Content-Type': 'application/json' },
+				body: JSON.stringify({ text }),
+				keepalive: true
+			});
 			pendingTexts.delete(blockId);
 		}
 	}
@@ -121,7 +124,7 @@ export function createBlockAutoSave({ saveFn, documentId }: Options) {
 		handleBlur,
 		handleRetry,
 		clearBlock,
-		flushViaBeacon,
+		flushOnUnload,
 		destroy
 	};
 }
diff --git a/frontend/src/routes/api/[...path]/+server.ts b/frontend/src/routes/api/[...path]/+server.ts
new file mode 100644
index 00000000..fa08d455
--- /dev/null
+++ b/frontend/src/routes/api/[...path]/+server.ts
@@ -0,0 +1,87 @@
+import type { RequestHandler } from './$types';
+import { env } from 'process';
+
+const NO_BODY_METHODS = new Set(['GET', 'HEAD', 'DELETE']);
+
+// Hop-by-hop headers must not be forwarded to the client — they are connection-scoped.
+const HOP_BY_HOP_HEADERS = new Set([
+	'transfer-encoding',
+	'connection',
+	'keep-alive',
+	'upgrade',
+	'proxy-authenticate',
+	'proxy-authorization',
+	'te',
+	'trailer',
+	'proxy-connection'
+]);
+
+async function proxy(event: Parameters<RequestHandler>[0]): Promise<Response> {
+	const rawPath = event.params.path;
+
+	// Block path traversal attempts — SvelteKit decodes %2F before routing,
+	// so encoded dots could produce ../../../ sequences after normalization.
+	if (rawPath.includes('..') || /%(2e)/i.test(rawPath)) {
+		return new Response('Bad Request', { status: 400 });
+	}
+
+	// Block Actuator endpoints — heapdump, env, configprops expose credentials
+	// and heap memory and must never be reachable through the SvelteKit proxy.
+	if (rawPath.startsWith('actuator')) {
+		return new Response('Not Found', { status: 404 });
+	}
+
+	const apiUrl = env.API_INTERNAL_URL || 'http://localhost:8080';
+	const backendUrl = `${apiUrl}/api/${rawPath}${event.url.search}`;
+
+	const hasBody = !NO_BODY_METHODS.has(event.request.method);
+
+	// Early exit: reject well-behaved clients that advertise an oversized body
+	// before we buffer anything.
+	const contentLength = event.request.headers.get('Content-Length');
+	if (contentLength && parseInt(contentLength, 10) > 1_048_576) {
+		return new Response('Payload Too Large', { status: 413 });
+	}
+
+	// Buffer the body so we can check its actual size. This also catches chunked
+	// requests that omit Content-Length entirely (parseInt(null) → NaN → passes
+	// the header check above).
+	const bodyBuffer = hasBody ? await event.request.arrayBuffer() : undefined;
+	if (bodyBuffer && bodyBuffer.byteLength > 1_048_576) {
+		return new Response('Payload Too Large', { status: 413 });
+	}
+
+	// Only forward Content-Type from the browser request — other headers (Cookie,
+	// Authorization, Accept-Encoding, etc.) must not be forwarded directly.
+	// Authentication is injected by handleFetch in hooks.server.ts from the
+	// server-side auth_token cookie.
+	const contentType = event.request.headers.get('Content-Type');
+	const requestHeaders: Record<string, string> = {};
+	if (contentType) requestHeaders['Content-Type'] = contentType;
+
+	const response = await event.fetch(backendUrl, {
+		method: event.request.method,
+		headers: requestHeaders,
+		body: bodyBuffer
+	});
+
+	// Forward all response headers except hop-by-hop so that Content-Disposition,
+	// ETag, Cache-Control, Location (redirects), etc. reach the browser.
+	const responseHeaders: Record<string, string> = {};
+	response.headers.forEach((value, key) => {
+		if (!HOP_BY_HOP_HEADERS.has(key.toLowerCase())) {
+			responseHeaders[key] = value;
+		}
+	});
+
+	return new Response(response.body, {
+		status: response.status,
+		headers: responseHeaders
+	});
+}
+
+export const GET: RequestHandler = (event) => proxy(event);
+export const POST: RequestHandler = (event) => proxy(event);
+export const PUT: RequestHandler = (event) => proxy(event);
+export const PATCH: RequestHandler = (event) => proxy(event);
+export const DELETE: RequestHandler = (event) => proxy(event);
diff --git a/frontend/src/routes/api/[...path]/proxy.spec.ts b/frontend/src/routes/api/[...path]/proxy.spec.ts
new file mode 100644
index 00000000..2831794c
--- /dev/null
+++ b/frontend/src/routes/api/[...path]/proxy.spec.ts
@@ -0,0 +1,221 @@
+import { describe, it, expect, vi } from 'vitest';
+
+process.env.API_INTERNAL_URL = 'http://backend:8080';
+
+const { GET, PUT, POST, PATCH } = await import('./+server');
+
+function makeEvent(
+	path: string,
+	method = 'GET',
+	mockFetch = vi
+		.fn()
+		.mockResolvedValue(
+			new Response('{}', { status: 200, headers: { 'Content-Type': 'application/json' } })
+		)
+) {
+	return {
+		params: { path },
+		request: new Request(`http://localhost/api/${path}`, { method }),
+		url: new URL(`http://localhost/api/${path}`),
+		fetch: mockFetch
+	};
+}
+
+describe('catch-all API proxy — security guards', () => {
+	it('returns 400 for path traversal with double-dot', async () => {
+		const event = makeEvent('documents/../../../etc/passwd');
+		const response = await GET(event as never);
+		expect(response.status).toBe(400);
+	});
+
+	it('returns 400 for URL-encoded dot traversal (%2e)', async () => {
+		const event = makeEvent('documents/%2e%2e/sensitive');
+		const response = await GET(event as never);
+		expect(response.status).toBe(400);
+	});
+
+	it('returns 400 for mixed-case encoded traversal (%2E)', async () => {
+		const event = makeEvent('documents/%2E%2E/sensitive');
+		const response = await GET(event as never);
+		expect(response.status).toBe(400);
+	});
+
+	it('returns 404 for actuator paths', async () => {
+		const event = makeEvent('actuator/heapdump');
+		const response = await GET(event as never);
+		expect(response.status).toBe(404);
+	});
+
+	it('returns 404 for actuator/env', async () => {
+		const event = makeEvent('actuator/env');
+		const response = await GET(event as never);
+		expect(response.status).toBe(404);
+	});
+});
+
+describe('catch-all API proxy — body size limit', () => {
+	it('returns 413 when Content-Length exceeds 1 MB', async () => {
+		const mockFetch = vi.fn();
+		const event = makeEvent('documents', 'POST', mockFetch);
+		(event.request as Request) = new Request('http://localhost/api/documents', {
+			method: 'POST',
+			headers: { 'Content-Type': 'application/json', 'Content-Length': '1048577' },
+			body: JSON.stringify({})
+		});
+
+		const response = await POST(event as never);
+
+		expect(response.status).toBe(413);
+		expect(mockFetch).not.toHaveBeenCalled();
+	});
+
+	it('returns 413 when actual body exceeds 1 MB even if Content-Length header is absent or lying', async () => {
+		const mockFetch = vi.fn();
+		const event = makeEvent('documents', 'POST', mockFetch);
+		(event.request as Request) = new Request('http://localhost/api/documents', {
+			method: 'POST',
+			headers: { 'Content-Type': 'application/octet-stream', 'Content-Length': '0' },
+			body: new Uint8Array(1_048_577) // 1 MB + 1 byte, but Content-Length says 0
+		});
+
+		const response = await POST(event as never);
+
+		expect(response.status).toBe(413);
+		expect(mockFetch).not.toHaveBeenCalled();
+	});
+
+	it('forwards request when Content-Length header is absent', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(new Response('{}', { status: 200 }));
+		const event = makeEvent('documents', 'POST', mockFetch);
+		(event.request as Request) = new Request('http://localhost/api/documents', {
+			method: 'POST',
+			headers: { 'Content-Type': 'application/json' },
+			body: JSON.stringify({ title: 'hello' })
+		});
+
+		const response = await POST(event as never);
+
+		expect(response.status).toBe(200);
+		expect(mockFetch).toHaveBeenCalled();
+	});
+
+	it('does not reject request with Content-Length exactly at 1 MB', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(new Response('{}', { status: 200 }));
+		const event = makeEvent('documents', 'POST', mockFetch);
+		(event.request as Request) = new Request('http://localhost/api/documents', {
+			method: 'POST',
+			headers: { 'Content-Type': 'application/json', 'Content-Length': '1048576' },
+			body: JSON.stringify({})
+		});
+
+		const response = await POST(event as never);
+
+		expect(response.status).toBe(200);
+	});
+});
+
+describe('catch-all API proxy — forwarding', () => {
+	it('forwards PUT request to backend with correct URL', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(
+			new Response('{"id":"block-1"}', {
+				status: 200,
+				headers: { 'Content-Type': 'application/json' }
+			})
+		);
+		const event = makeEvent('documents/doc-1/transcription-blocks/block-1', 'PUT', mockFetch);
+		(event.request as Request) = new Request(
+			'http://localhost/api/documents/doc-1/transcription-blocks/block-1',
+			{
+				method: 'PUT',
+				headers: { 'Content-Type': 'application/json' },
+				body: JSON.stringify({ text: 'hello' })
+			}
+		);
+
+		const response = await PUT(event as never);
+
+		expect(mockFetch).toHaveBeenCalledWith(
+			'http://backend:8080/api/documents/doc-1/transcription-blocks/block-1',
+			expect.objectContaining({ method: 'PUT' })
+		);
+		expect(response.status).toBe(200);
+	});
+
+	it('passes through the backend response status code', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(new Response(null, { status: 404 }));
+		const event = makeEvent('documents/nonexistent', 'GET', mockFetch);
+
+		const response = await GET(event as never);
+
+		expect(response.status).toBe(404);
+	});
+
+	it('forwards Content-Disposition response header', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(
+			new Response('data', {
+				status: 200,
+				headers: {
+					'Content-Type': 'application/pdf',
+					'Content-Disposition': 'attachment; filename="document.pdf"'
+				}
+			})
+		);
+		const event = makeEvent('documents/doc-1/export', 'GET', mockFetch);
+
+		const response = await GET(event as never);
+
+		expect(response.headers.get('Content-Disposition')).toBe('attachment; filename="document.pdf"');
+	});
+
+	it('does not forward proxy-connection hop-by-hop header', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(
+			new Response('data', {
+				status: 200,
+				headers: {
+					'Content-Type': 'application/json',
+					'Proxy-Connection': 'keep-alive'
+				}
+			})
+		);
+		const event = makeEvent('documents', 'GET', mockFetch);
+
+		const response = await GET(event as never);
+
+		expect(response.headers.get('Proxy-Connection')).toBeNull();
+	});
+
+	it('does not forward hop-by-hop headers like transfer-encoding', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(
+			new Response('data', {
+				status: 200,
+				headers: {
+					'Content-Type': 'application/json',
+					'Transfer-Encoding': 'chunked'
+				}
+			})
+		);
+		const event = makeEvent('documents', 'GET', mockFetch);
+
+		const response = await GET(event as never);
+
+		expect(response.headers.get('Transfer-Encoding')).toBeNull();
+	});
+
+	it('forwards PATCH request to backend with correct URL', async () => {
+		const mockFetch = vi.fn().mockResolvedValue(new Response('{}', { status: 200 }));
+		const event = makeEvent('documents/doc-1', 'PATCH', mockFetch);
+		(event.request as Request) = new Request('http://localhost/api/documents/doc-1', {
+			method: 'PATCH',
+			headers: { 'Content-Type': 'application/json' },
+			body: JSON.stringify({ title: 'updated' })
+		});
+
+		const response = await PATCH(event as never);
+
+		expect(mockFetch).toHaveBeenCalledWith(
+			'http://backend:8080/api/documents/doc-1',
+			expect.objectContaining({ method: 'PATCH' })
+		);
+		expect(response.status).toBe(200);
+	});
+});