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/config/config.go

@@ -50,13 +50,20 @@ type MinIO struct {
 
 // Kokoro holds connection settings for the Kokoro-FastAPI TTS service.
 type Kokoro struct {
-	// URL is the base URL of the Kokoro service, e.g. https://kokoro.libnovel.cc
-	// An empty string disables TTS generation.
+	// URL is the base URL of the Kokoro service, e.g. https://tts.libnovel.cc
+	// An empty string disables Kokoro TTS generation.
 	URL string
 	// DefaultVoice is the voice used when none is specified.
 	DefaultVoice string
 }
 
+// PocketTTS holds connection settings for the kyutai-labs/pocket-tts service.
+type PocketTTS struct {
+	// URL is the base URL of the pocket-tts service, e.g. https://pocket-tts.libnovel.cc
+	// An empty string disables pocket-tts generation.
+	URL string
+}
+
 // HTTP holds settings for the HTTP server (backend only).
 type HTTP struct {
 	// Addr is the listen address, e.g. ":8080"
@@ -113,6 +120,7 @@ type Config struct {
 	PocketBase  PocketBase
 	MinIO       MinIO
 	Kokoro      Kokoro
+	PocketTTS   PocketTTS
 	HTTP        HTTP
 	Runner      Runner
 	Meilisearch Meilisearch
@@ -156,6 +164,10 @@ func Load() Config {
 			DefaultVoice: envOr("KOKORO_VOICE", "af_bella"),
 		},
 
+		PocketTTS: PocketTTS{
+			URL: envOr("POCKET_TTS_URL", ""),
+		},
+
 		HTTP: HTTP{
 			Addr: envOr("BACKEND_HTTP_ADDR", ":8080"),
 		},