feat(audio): WAV streaming, bulk audio generation admin endpoints, cancel/resume

- Add StreamAudioWAV() to pocket-tts and Kokoro clients; pocket-tts streams
  raw WAV directly (no ffmpeg), Kokoro requests response_format:wav with stream:true
- GET /api/audio-stream supports ?format=wav for lower-latency first-byte delivery;
  WAV cached separately in MinIO as {slug}/{n}/{voice}.wav
- Add GET /api/admin/audio/jobs with optional ?slug filter
- Add POST /api/admin/audio/bulk {slug, voice, from, to, skip_existing, force}
  where skip_existing=true (default) resumes interrupted bulk jobs
- Add POST /api/admin/audio/cancel-bulk {slug} to cancel all pending/running tasks
- Add CancelAudioTasksBySlug to taskqueue.Producer + asynqqueue implementation
- Add AudioObjectKeyExt to bookstore.AudioStore for format-aware MinIO keys

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/internal/asynqqueue/producer.go

@@ -93,6 +93,12 @@ func (p *Producer) CancelTask(ctx context.Context, id string) error {
 	return p.pb.CancelTask(ctx, id)
 }
 
+// CancelAudioTasksBySlug delegates to PocketBase to cancel all pending/running
+// audio tasks for slug.
+func (p *Producer) CancelAudioTasksBySlug(ctx context.Context, slug string) (int, error) {
+	return p.pb.CancelAudioTasksBySlug(ctx, slug)
+}
+
 // enqueue serialises payload and dispatches it to Asynq.
 func (p *Producer) enqueue(_ context.Context, taskType string, payload any) error {
 	b, err := json.Marshal(payload)