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/lib/server/pocketbase.ts

@@ -37,7 +37,9 @@ export interface ChapterIdx {
 }
 
 export interface Progress {
+	id?: string;
 	session_id: string;
+	user_id?: string;
 	slug: string;
 	chapter: number;
 	updated: string;
@@ -160,26 +162,51 @@ export async function listChapterIdx(slug: string): Promise<ChapterIdx[]> {
 
 // ─── Reading progress ─────────────────────────────────────────────────────────
 
-export async function getProgress(sessionId: string, slug: string): Promise<Progress | null> {
-	return listOne<Progress>('progress', `session_id="${sessionId}"&&slug="${slug}"`);
+/**
+ * Build the PocketBase filter string for a progress lookup.
+ * When userId is set, keyed by user_id (portable across devices).
+ * When only sessionId is set, keyed by session_id (anonymous).
+ */
+function progressFilter(sessionId: string, slug: string, userId?: string): string {
+	if (userId) return `user_id="${userId}"&&slug="${slug}"`;
+	return `session_id="${sessionId}"&&slug="${slug}"`;
 }
 
-export async function allProgress(sessionId: string): Promise<Progress[]> {
-	return listAll<Progress>('progress', `session_id="${sessionId}"`, '-updated');
+function allProgressFilter(sessionId: string, userId?: string): string {
+	if (userId) return `user_id="${userId}"`;
+	return `session_id="${sessionId}"`;
 }
 
-export async function setProgress(sessionId: string, slug: string, chapter: number): Promise<void> {
+export async function getProgress(
+	sessionId: string,
+	slug: string,
+	userId?: string
+): Promise<Progress | null> {
+	return listOne<Progress>('progress', progressFilter(sessionId, slug, userId));
+}
+
+export async function allProgress(sessionId: string, userId?: string): Promise<Progress[]> {
+	return listAll<Progress>('progress', allProgressFilter(sessionId, userId), '-updated');
+}
+
+export async function setProgress(
+	sessionId: string,
+	slug: string,
+	chapter: number,
+	userId?: string
+): Promise<void> {
 	const existing = await listOne<Progress & { id: string }>(
 		'progress',
-		`session_id="${sessionId}"&&slug="${slug}"`
+		progressFilter(sessionId, slug, userId)
 	);
 
-	const payload = {
+	const payload: Partial<Progress> = {
 		session_id: sessionId,
 		slug,
 		chapter,
 		updated: new Date().toISOString()
 	};
+	if (userId) payload.user_id = userId;
 
 	if (existing) {
 		const res = await pbPatch(`/api/collections/progress/records/${existing.id}`, payload);
@@ -196,6 +223,63 @@ export async function setProgress(sessionId: string, slug: string, chapter: numb
 	}
 }
 
+/**
+ * Merge anonymous session progress into a user account on login/register.
+ *
+ * For each book tracked under sessionId, upserts a user-keyed record keeping
+ * whichever chapter is more recent (or higher if timestamps are equal).
+ * This makes progress portable across devices for logged-in users.
+ */
+export async function mergeSessionProgress(sessionId: string, userId: string): Promise<void> {
+	let sessionRows: Progress[];
+	try {
+		sessionRows = await allProgress(sessionId);
+	} catch (e) {
+		log.warn('pocketbase', 'mergeSessionProgress: failed to read session progress', {
+			sessionId,
+			err: String(e)
+		});
+		return;
+	}
+	if (sessionRows.length === 0) return;
+
+	for (const row of sessionRows) {
+		try {
+			const userRow = await listOne<Progress & { id: string }>(
+				'progress',
+				`user_id="${userId}"&&slug="${row.slug}"`
+			);
+
+			// Keep the record with the more recent update (or higher chapter if timestamps match)
+			const sessionTs = row.updated ? new Date(row.updated).getTime() : 0;
+			const userTs = userRow?.updated ? new Date(userRow.updated).getTime() : 0;
+			const shouldOverwrite = !userRow || sessionTs > userTs ||
+				(sessionTs === userTs && row.chapter > (userRow?.chapter ?? 0));
+
+			if (shouldOverwrite) {
+				const payload: Partial<Progress> = {
+					session_id: sessionId,
+					user_id: userId,
+					slug: row.slug,
+					chapter: row.chapter,
+					updated: row.updated ?? new Date().toISOString()
+				};
+				if (userRow) {
+					await pbPatch(`/api/collections/progress/records/${userRow.id}`, payload);
+				} else {
+					await pbPost('/api/collections/progress/records', payload);
+				}
+			}
+		} catch (e) {
+			log.warn('pocketbase', 'mergeSessionProgress: failed to merge row', {
+				slug: row.slug,
+				err: String(e)
+			});
+		}
+	}
+	log.info('pocketbase', 'mergeSessionProgress: done', { sessionId, userId, count: sessionRows.length });
+}
+
 // ─── Users ────────────────────────────────────────────────────────────────────
 
 import { scryptSync, randomBytes, timingSafeEqual } from 'node:crypto';