fix(transcription): replace sendBeacon with fetch keepalive; add catch-all API proxy

sendBeacon always sends POST, but the backend expects PUT for block updates, so
saves were silently dropped on page unload.  Replace with fetch({ keepalive: true,
method: 'PUT' }) which survives navigation and uses the correct HTTP method.

Add a catch-all SvelteKit server route at /api/[...path] so all client-side API
calls work in production (without the Vite dev proxy).  More-specific routes
(/api/persons, /api/tags, /api/documents/[id]/file) keep precedence.

Closes #204

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

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 +82,70 @@ describe('createBlockAutoSave', () => {
 		expect(mockSaveFn).not.toHaveBeenCalled();
 	});
 });
+
+describe('flushViaBeacon', () => {
+	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.flushViaBeacon();
+
+		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.flushViaBeacon();
+
+		expect(sendBeaconSpy).not.toHaveBeenCalled();
+	});
+
+	it('does nothing when there are no pending edits', () => {
+		const as = createBlockAutoSave({ saveFn: mockSaveFn, documentId: 'doc-1' });
+		as.flushViaBeacon();
+
+		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.flushViaBeacon();
+
+		await vi.advanceTimersByTimeAsync(2000);
+		expect(mockSaveFn).not.toHaveBeenCalled();
+	});
+});