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/runner/runner_test.go

@@ -126,6 +126,9 @@ type stubAudioStore struct {
 func (s *stubAudioStore) AudioObjectKey(slug string, n int, voice string) string {
 	return slug + "/" + string(rune('0'+n)) + "/" + voice + ".mp3"
 }
+func (s *stubAudioStore) AudioObjectKeyExt(slug string, n int, voice, ext string) string {
+	return slug + "/" + string(rune('0'+n)) + "/" + voice + "." + ext
+}
 func (s *stubAudioStore) AudioExists(_ context.Context, _ string) bool { return false }
 func (s *stubAudioStore) PutAudio(_ context.Context, _ string, _ []byte) error {
 	s.putCalled.Add(1)
@@ -199,6 +202,14 @@ func (s *stubKokoro) StreamAudioMP3(_ context.Context, _, _ string) (io.ReadClos
 	return io.NopCloser(bytes.NewReader(s.data)), nil
 }
 
+func (s *stubKokoro) StreamAudioWAV(_ context.Context, _, _ string) (io.ReadCloser, error) {
+	s.called.Add(1)
+	if s.genErr != nil {
+		return nil, s.genErr
+	}
+	return io.NopCloser(bytes.NewReader(s.data)), nil
+}
+
 func (s *stubKokoro) ListVoices(_ context.Context) ([]string, error) {
 	return []string{"af_bella"}, nil
 }