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/api/admin/audio-jobs/+server.ts

@@ -0,0 +1,16 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { listAudioJobs } from '$lib/server/pocketbase';
+
+/**
+ * GET /api/admin/audio-jobs
+ * Returns the current audio jobs list (served from 30 s Valkey cache).
+ * Used by the admin audio page for lightweight polling instead of invalidateAll().
+ */
+export const GET: RequestHandler = async ({ locals }) => {
+	if (!locals.user || locals.user.role !== 'admin') {
+		throw error(403, 'Forbidden');
+	}
+	const jobs = await listAudioJobs().catch(() => []);
+	return json({ jobs });
+};

ui/src/routes/api/admin/scrape-tasks/+server.ts

@@ -0,0 +1,16 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { listScrapingTasks } from '$lib/server/pocketbase';
+
+/**
+ * GET /api/admin/scrape-tasks
+ * Returns the current scraping task list (served from 30 s Valkey cache).
+ * Used by the admin scrape page for lightweight polling instead of invalidateAll().
+ */
+export const GET: RequestHandler = async ({ locals }) => {
+	if (!locals.user || locals.user.role !== 'admin') {
+		throw error(403, 'Forbidden');
+	}
+	const tasks = await listScrapingTasks().catch(() => []);
+	return json({ tasks });
+};

ui/src/routes/api/admin/translation-jobs/+server.ts

@@ -0,0 +1,16 @@
+import { json, error } from '@sveltejs/kit';
+import type { RequestHandler } from './$types';
+import { listTranslationJobs } from '$lib/server/pocketbase';
+
+/**
+ * GET /api/admin/translation-jobs
+ * Returns the current translation jobs list (served from 30 s Valkey cache).
+ * Used by the admin translation page for lightweight polling instead of invalidateAll().
+ */
+export const GET: RequestHandler = async ({ locals }) => {
+	if (!locals.user || locals.user.role !== 'admin') {
+		throw error(403, 'Forbidden');
+	}
+	const jobs = await listTranslationJobs().catch(() => []);
+	return json({ jobs });
+};