perf: cache admin job lists + targeted polling for audio/translation pages

- Add 30s Valkey cache to listScrapingTasks, listAudioJobs, listTranslationJobs
  (use listN(500) instead of unbounded listAll to cap at one request)
- Delete listAudioCache() — derive AudioCacheEntry[] from jobs in server load
- Add listBookSlugs() with 10min cache — replaces full listBooks() in translation load
- Add GET /api/admin/audio-jobs, /api/admin/translation-jobs, /api/admin/scrape-tasks
  (lightweight polling endpoints backed by the Valkey cache)
- Replace invalidateAll() interval polling in audio+translation pages with
  targeted fetch to the new endpoints (avoids re-running full server load)

ui/src/lib/server/pocketbase.ts

@@ -248,6 +248,14 @@ const RATINGS_CACHE_TTL = 5 * 60; // 5 minutes
 const HOME_STATS_CACHE_KEY = 'home:stats';
 const HOME_STATS_CACHE_TTL = 10 * 60; // 10 minutes — counts don't need to be exact
 
+const SCRAPING_TASKS_CACHE_KEY = 'admin:scraping_tasks';
+const AUDIO_JOBS_CACHE_KEY = 'admin:audio_jobs';
+const TRANSLATION_JOBS_CACHE_KEY = 'admin:translation_jobs';
+const ADMIN_JOBS_CACHE_TTL = 30; // 30 seconds — admin views poll frequently
+
+const BOOK_SLUGS_CACHE_KEY = 'books:slugs';
+const BOOK_SLUGS_CACHE_TTL = 10 * 60; // 10 minutes — slugs change rarely
+
 async function getAllRatings(): Promise<BookRating[]> {
 	const cached = await cache.get<BookRating[]>(RATINGS_CACHE_KEY);
 	if (cached) return cached;
@@ -272,6 +280,31 @@ export async function listBooks(): Promise<Book[]> {
 	return books;
 }
 
+export interface BookSlug {
+	slug: string;
+	title: string;
+}
+
+/**
+ * Returns only the slug and title of every book. Cheaper than listBooks() —
+ * used for datalist autocomplete in admin forms. Cached for 10 minutes.
+ */
+export async function listBookSlugs(): Promise<BookSlug[]> {
+	const cached = await cache.get<BookSlug[]>(BOOK_SLUGS_CACHE_KEY);
+	if (cached) return cached;
+	// Re-use full books cache if already warm — avoids a second PocketBase call.
+	const fullCached = await cache.get<Book[]>(BOOKS_CACHE_KEY);
+	if (fullCached) {
+		const slugs = fullCached.map((b) => ({ slug: b.slug, title: b.title }));
+		await cache.set(BOOK_SLUGS_CACHE_KEY, slugs, BOOK_SLUGS_CACHE_TTL);
+		return slugs;
+	}
+	const items = await listAll<BookSlug>('books', '', '+title').catch(() => [] as BookSlug[]);
+	const slugs = items.map((b) => ({ slug: b.slug, title: b.title }));
+	await cache.set(BOOK_SLUGS_CACHE_KEY, slugs, BOOK_SLUGS_CACHE_TTL);
+	return slugs;
+}
+
 /**
  * Fetch only the books whose slugs are in the given set.
  * Uses PocketBase filter `slug IN (...)` — a single request regardless of how
@@ -1052,16 +1085,6 @@ export interface AudioCacheEntry {
 	updated: string;
 }
 
-export async function listAudioCache(): Promise<AudioCacheEntry[]> {
-	const jobs = await listAll<AudioJob>('audio_jobs', 'status="done"', '-finished');
-	return jobs.map((j) => ({
-		id: j.id,
-		cache_key: j.cache_key,
-		filename: `${j.cache_key}.mp3`,
-		updated: j.finished
-	}));
-}
-
 // ─── Scraping tasks ───────────────────────────────────────────────────────────
 
 export interface ScrapingTask {
@@ -1081,7 +1104,11 @@ export interface ScrapingTask {
 }
 
 export async function listScrapingTasks(): Promise<ScrapingTask[]> {
-	return listAll<ScrapingTask>('scraping_tasks', '', '-started');
+	const cached = await cache.get<ScrapingTask[]>(SCRAPING_TASKS_CACHE_KEY);
+	if (cached) return cached;
+	const tasks = await listN<ScrapingTask>('scraping_tasks', 500, '', '-started');
+	await cache.set(SCRAPING_TASKS_CACHE_KEY, tasks, ADMIN_JOBS_CACHE_TTL);
+	return tasks;
 }
 
 export async function getScrapingTask(id: string): Promise<ScrapingTask | null> {
@@ -1103,7 +1130,11 @@ export interface AudioJob {
 }
 
 export async function listAudioJobs(): Promise<AudioJob[]> {
-	return listAll<AudioJob>('audio_jobs', '', '-started');
+	const cached = await cache.get<AudioJob[]>(AUDIO_JOBS_CACHE_KEY);
+	if (cached) return cached;
+	const jobs = await listN<AudioJob>('audio_jobs', 500, '', '-started');
+	await cache.set(AUDIO_JOBS_CACHE_KEY, jobs, ADMIN_JOBS_CACHE_TTL);
+	return jobs;
 }
 
 /**
@@ -1130,7 +1161,11 @@ export interface TranslationJob {
 }
 
 export async function listTranslationJobs(): Promise<TranslationJob[]> {
-	return listAll<TranslationJob>('translation_jobs', '', '-started');
+	const cached = await cache.get<TranslationJob[]>(TRANSLATION_JOBS_CACHE_KEY);
+	if (cached) return cached;
+	const jobs = await listN<TranslationJob>('translation_jobs', 500, '', '-started');
+	await cache.set(TRANSLATION_JOBS_CACHE_KEY, jobs, ADMIN_JOBS_CACHE_TTL);
+	return jobs;
 }
 
 export async function getAudioTime(