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/taskqueue/taskqueue.go

@@ -36,6 +36,10 @@ type Producer interface {
 	// CancelTask transitions a pending task to status=cancelled.
 	// Returns ErrNotFound if the task does not exist.
 	CancelTask(ctx context.Context, id string) error
+
+	// CancelAudioTasksBySlug cancels all pending or running audio tasks for slug.
+	// Returns the number of tasks cancelled.
+	CancelAudioTasksBySlug(ctx context.Context, slug string) (int, error)
 }
 
 // Consumer is the read/claim side of the task queue used by the runner.