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/useBlockAutoSave.svelte.ts

@@ -97,9 +97,12 @@ export function createBlockAutoSave({ saveFn, documentId }: Options) {
 	function flushViaBeacon(): 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' }));
+			fetch(`/api/documents/${documentId}/transcription-blocks/${blockId}`, {
+				method: 'PUT',
+				headers: { 'Content-Type': 'application/json' },
+				body: JSON.stringify({ text }),
+				keepalive: true
+			});
 			pendingTexts.delete(blockId);
 		}
 	}