feat(tts): add pocket-tts engine alongside kokoro-fastapi

- New backend/internal/pockettts package: POST /tts client for
  kyutai-labs/pocket-tts; streams WAV response and transcodes to MP3
  via ffmpeg so the rest of the pipeline stays format-agnostic
- config.PocketTTS struct + POCKET_TTS_URL env var
- runner.Dependencies.PocketTTS field; runAudioTask routes by voice name:
  pockettts.IsPocketTTSVoice (alba, marius, javert, …) → pocket-tts,
  everything else → kokoro-fastapi
- Dockerfile: runner stage switched from distroless/static to alpine:3.21
  with ffmpeg + ca-certificates installed
- homelab compose: runner gets POCKET_TTS_URL=http://pocket-tts:8000
- Doppler prd + prd_homelab: KOKORO_URL=https://tts.libnovel.cc,
  POCKET_TTS_URL=https://pocket-tts.libnovel.cc

backend/internal/runner/runner.go

@@ -31,6 +31,7 @@ import (
 	"github.com/libnovel/backend/internal/kokoro"
 	"github.com/libnovel/backend/internal/meili"
 	"github.com/libnovel/backend/internal/orchestrator"
+	"github.com/libnovel/backend/internal/pockettts"
 	"github.com/libnovel/backend/internal/scraper"
 	"github.com/libnovel/backend/internal/taskqueue"
 )
@@ -84,8 +85,11 @@ type Dependencies struct {
 	SearchIndex meili.Client
 	// Novel is the scraper implementation.
 	Novel scraper.NovelScraper
-	// Kokoro is the TTS client.
+	// Kokoro is the Kokoro-FastAPI TTS client (GPU, OpenAI-compatible voices).
 	Kokoro kokoro.Client
+	// PocketTTS is the pocket-tts client (CPU, kyutai voices: alba, marius, etc.).
+	// If nil, pocket-tts voice tasks will fail with a clear error.
+	PocketTTS pockettts.Client
 	// Log is the structured logger.
 	Log *slog.Logger
 }
@@ -448,14 +452,31 @@ func (r *Runner) runAudioTask(ctx context.Context, task domain.AudioTask) {
 		return
 	}
 
-	if r.deps.Kokoro == nil {
-		fail("kokoro client not configured")
-		return
-	}
-	audioData, err := r.deps.Kokoro.GenerateAudio(ctx, text, task.Voice)
-	if err != nil {
-		fail(fmt.Sprintf("kokoro generate: %v", err))
-		return
+	var audioData []byte
+	if pockettts.IsPocketTTSVoice(task.Voice) {
+		if r.deps.PocketTTS == nil {
+			fail("pocket-tts client not configured (POCKET_TTS_URL is empty)")
+			return
+		}
+		var genErr error
+		audioData, genErr = r.deps.PocketTTS.GenerateAudio(ctx, text, task.Voice)
+		if genErr != nil {
+			fail(fmt.Sprintf("pocket-tts generate: %v", genErr))
+			return
+		}
+		log.Info("runner: audio generated via pocket-tts", "voice", task.Voice)
+	} else {
+		if r.deps.Kokoro == nil {
+			fail("kokoro client not configured (KOKORO_URL is empty)")
+			return
+		}
+		var genErr error
+		audioData, genErr = r.deps.Kokoro.GenerateAudio(ctx, text, task.Voice)
+		if genErr != nil {
+			fail(fmt.Sprintf("kokoro generate: %v", genErr))
+			return
+		}
+		log.Info("runner: audio generated via kokoro-fastapi", "voice", task.Voice)
 	}
 
 	key := r.deps.AudioStore.AudioObjectKey(task.Slug, task.Chapter, task.Voice)