Add async audio generation: job tracking in PocketBase + UI polling

Replace blocking POST /api/audio with a non-blocking 202 flow: the Go
handler immediately enqueues a job in a new `audio_jobs` PocketBase
collection and returns {job_id, status}. A background goroutine runs
the actual Kokoro TTS work and updates job status (pending → generating
→ done/failed). A new GET /api/audio/status/{slug}/{n} endpoint lets
clients poll progress. The SvelteKit proxy and AudioPlayer.svelte are
updated to POST, then poll the status route every 2s until done.

scraper/internal/orchestrator/orchestrator_test.go

@@ -173,6 +173,18 @@ func (s *mockStore) UpdateScrapeTask(_ context.Context, _ string, _ storage.Scra
 func (s *mockStore) ListScrapeTasks(_ context.Context) ([]storage.ScrapeTask, error) {
 	return nil, nil
 }
+func (s *mockStore) CreateAudioJob(_ context.Context, _ string, _ int, _ string) (string, error) {
+	return "audio-job-id", nil
+}
+func (s *mockStore) UpdateAudioJob(_ context.Context, _, _, _ string, _ time.Time) error {
+	return nil
+}
+func (s *mockStore) GetAudioJob(_ context.Context, _ string) (storage.AudioJob, bool, error) {
+	return storage.AudioJob{}, false, nil
+}
+func (s *mockStore) ListAudioJobs(_ context.Context) ([]storage.AudioJob, error) {
+	return nil, nil
+}
 
 // ── helpers ───────────────────────────────────────────────────────────────────