feat(progress): associate progress with logged-in user for cross-device sync

Add optional user_id field to the progress collection. When a user is
authenticated, progress is keyed by user_id instead of session_id, making
it portable across devices and browsers. Anonymous reading still works via
session_id as before.

On login or registration, any progress accumulated under the anonymous
session is merged into the user's account (most-recent timestamp wins),
so chapters read before logging in are not lost.

- scripts/pb-init.sh: add user_id field to progress collection schema
- scraper/internal/storage/pocketbase.go: add user_id to EnsureCollections
- ui/src/lib/server/pocketbase.ts: Progress interface gains user_id;
  getProgress/allProgress/setProgress accept optional userId and query/write
  by user_id when present; add mergeSessionProgress() helper
- ui/src/routes/login/+page.server.ts: call mergeSessionProgress after
  successful login and registration (fire-and-forget, non-fatal)
- ui/src/routes/api/progress/+server.ts: pass locals.user?.id to setProgress
- ui/src/routes/books/+page.server.ts: pass locals.user?.id to allProgress
- ui/src/routes/books/[slug]/+page.server.ts: pass locals.user?.id to getProgress

ui/src/routes/api/progress/+server.ts

@@ -6,7 +6,9 @@ import { log } from '$lib/server/logger';
 /**
  * POST /api/progress
  * Body: { slug: string, chapter: number }
- * Records the user's reading position for the current session.
+ * Records the user's reading position.
+ * When the user is logged in, progress is keyed by user_id so it syncs across devices.
+ * When anonymous, progress is keyed by the session cookie.
  */
 export const POST: RequestHandler = async ({ request, locals }) => {
 	const body = await request.json().catch(() => null);
@@ -16,7 +18,7 @@ export const POST: RequestHandler = async ({ request, locals }) => {
 	}
 
 	try {
-		await setProgress(locals.sessionId, body.slug, body.chapter);
+		await setProgress(locals.sessionId, body.slug, body.chapter, locals.user?.id);
 	} catch (e) {
 		log.error('progress', 'setProgress failed', { slug: body.slug, chapter: body.chapter, err: String(e) });
 		error(500, 'Failed to save progress');