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/cmd/runner/main.go

@@ -26,6 +26,7 @@ import (
 	"github.com/libnovel/backend/internal/meili"
 	"github.com/libnovel/backend/internal/novelfire"
 	"github.com/libnovel/backend/internal/otelsetup"
+	"github.com/libnovel/backend/internal/pockettts"
 	"github.com/libnovel/backend/internal/runner"
 	"github.com/libnovel/backend/internal/storage"
 )
@@ -112,10 +113,19 @@ func run() error {
 		kokoroClient = kokoro.New(cfg.Kokoro.URL)
 		log.Info("kokoro TTS enabled", "url", cfg.Kokoro.URL)
 	} else {
-		log.Warn("KOKORO_URL not set — audio tasks will fail")
+		log.Warn("KOKORO_URL not set — kokoro voice tasks will fail")
 		kokoroClient = &noopKokoro{}
 	}
 
+	// ── pocket-tts ──────────────────────────────────────────────────────────
+	var pocketTTSClient pockettts.Client
+	if cfg.PocketTTS.URL != "" {
+		pocketTTSClient = pockettts.New(cfg.PocketTTS.URL)
+		log.Info("pocket-tts enabled", "url", cfg.PocketTTS.URL)
+	} else {
+		log.Warn("POCKET_TTS_URL not set — pocket-tts voice tasks will fail")
+	}
+
 	// ── Meilisearch ─────────────────────────────────────────────────────────
 	var searchIndex meili.Client
 	if cfg.Meilisearch.URL != "" {
@@ -151,6 +161,7 @@ func run() error {
 		SearchIndex: searchIndex,
 		Novel:       novel,
 		Kokoro:      kokoroClient,
+		PocketTTS:   pocketTTSClient,
 		Log:         log,
 	}
 	r := runner.New(rCfg, deps)