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/routes/admin/translation/+page.server.ts

@@ -1,6 +1,6 @@
 import { redirect } from '@sveltejs/kit';
 import type { Actions, PageServerLoad } from './$types';
-import { listBooks, listTranslationJobs, type TranslationJob } from '$lib/server/pocketbase';
+import { listBookSlugs, listTranslationJobs, type TranslationJob } from '$lib/server/pocketbase';
 import { backendFetch } from '$lib/server/scraper';
 import { log } from '$lib/server/logger';
 
@@ -10,8 +10,8 @@ export const load: PageServerLoad = async ({ locals }) => {
 	}
 
 	const [books, jobs] = await Promise.all([
-		listBooks().catch((e): Awaited<ReturnType<typeof listBooks>> => {
-			log.warn('admin/translation', 'failed to load books', { err: String(e) });
+		listBookSlugs().catch((e): Awaited<ReturnType<typeof listBookSlugs>> => {
+			log.warn('admin/translation', 'failed to load book slugs', { err: String(e) });
 			return [];
 		}),
 		listTranslationJobs().catch((e): TranslationJob[] => {