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/pockettts/client.go

@@ -0,0 +1,159 @@
+// Package pockettts provides a client for the kyutai-labs/pocket-tts TTS service.
+//
+// pocket-tts exposes a non-OpenAI API:
+//
+//	POST /tts  (multipart form: text, voice_url)  → streaming WAV
+//	GET  /health                                   → {"status":"healthy"}
+//
+// GenerateAudio streams the WAV response and transcodes it to MP3 using ffmpeg,
+// so callers receive MP3 bytes — the same format as the kokoro client — and the
+// rest of the pipeline does not need to care which TTS engine was used.
+//
+// Predefined voices (pass the bare name as the voice parameter):
+//
+//	alba, marius, javert, jean, fantine, cosette, eponine, azelma,
+//	anna, vera, charles, paul, george, mary, jane, michael, eve,
+//	bill_boerst, peter_yearsley, stuart_bell
+package pockettts
+
+import (
+	"bytes"
+	"context"
+	"fmt"
+	"io"
+	"mime/multipart"
+	"net/http"
+	"os/exec"
+	"strings"
+	"time"
+)
+
+// PredefinedVoices is the set of voice names built into pocket-tts.
+// The runner uses this to decide which TTS engine to route a task to.
+var PredefinedVoices = map[string]struct{}{
+	"alba": {}, "marius": {}, "javert": {}, "jean": {},
+	"fantine": {}, "cosette": {}, "eponine": {}, "azelma": {},
+	"anna": {}, "vera": {}, "charles": {}, "paul": {},
+	"george": {}, "mary": {}, "jane": {}, "michael": {},
+	"eve": {}, "bill_boerst": {}, "peter_yearsley": {}, "stuart_bell": {},
+}
+
+// IsPocketTTSVoice reports whether voice is served by pocket-tts.
+func IsPocketTTSVoice(voice string) bool {
+	_, ok := PredefinedVoices[voice]
+	return ok
+}
+
+// Client is the interface for interacting with the pocket-tts service.
+type Client interface {
+	// GenerateAudio synthesises text using the given voice and returns MP3 bytes.
+	// Voice must be one of the predefined pocket-tts voice names.
+	GenerateAudio(ctx context.Context, text, voice string) ([]byte, error)
+
+	// ListVoices returns the available predefined voice names.
+	ListVoices(ctx context.Context) ([]string, error)
+}
+
+// httpClient is the concrete pocket-tts HTTP client.
+type httpClient struct {
+	baseURL string
+	http    *http.Client
+}
+
+// New returns a Client targeting baseURL (e.g. "https://pocket-tts.libnovel.cc").
+func New(baseURL string) Client {
+	return &httpClient{
+		baseURL: strings.TrimRight(baseURL, "/"),
+		http:    &http.Client{Timeout: 10 * time.Minute},
+	}
+}
+
+// GenerateAudio posts to POST /tts and transcodes the WAV response to MP3
+// using the system ffmpeg binary. Requires ffmpeg to be on PATH (available in
+// the runner Docker image via Alpine's ffmpeg package).
+func (c *httpClient) GenerateAudio(ctx context.Context, text, voice string) ([]byte, error) {
+	if text == "" {
+		return nil, fmt.Errorf("pockettts: empty text")
+	}
+	if voice == "" {
+		voice = "alba"
+	}
+
+	// ── Build multipart form ──────────────────────────────────────────────────
+	var body bytes.Buffer
+	mw := multipart.NewWriter(&body)
+
+	if err := mw.WriteField("text", text); err != nil {
+		return nil, fmt.Errorf("pockettts: write text field: %w", err)
+	}
+	// pocket-tts accepts a predefined voice name as voice_url.
+	if err := mw.WriteField("voice_url", voice); err != nil {
+		return nil, fmt.Errorf("pockettts: write voice_url field: %w", err)
+	}
+	if err := mw.Close(); err != nil {
+		return nil, fmt.Errorf("pockettts: close multipart writer: %w", err)
+	}
+
+	req, err := http.NewRequestWithContext(ctx, http.MethodPost,
+		c.baseURL+"/tts", &body)
+	if err != nil {
+		return nil, fmt.Errorf("pockettts: build request: %w", err)
+	}
+	req.Header.Set("Content-Type", mw.FormDataContentType())
+
+	resp, err := c.http.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("pockettts: request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != http.StatusOK {
+		_, _ = io.Copy(io.Discard, resp.Body)
+		return nil, fmt.Errorf("pockettts: server returned %d", resp.StatusCode)
+	}
+
+	wavData, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("pockettts: read response body: %w", err)
+	}
+
+	// ── Transcode WAV → MP3 via ffmpeg ────────────────────────────────────────
+	mp3Data, err := wavToMP3(ctx, wavData)
+	if err != nil {
+		return nil, fmt.Errorf("pockettts: transcode to mp3: %w", err)
+	}
+	return mp3Data, nil
+}
+
+// ListVoices returns the statically known predefined voice names.
+// pocket-tts has no REST endpoint for listing voices.
+func (c *httpClient) ListVoices(_ context.Context) ([]string, error) {
+	voices := make([]string, 0, len(PredefinedVoices))
+	for v := range PredefinedVoices {
+		voices = append(voices, v)
+	}
+	return voices, nil
+}
+
+// wavToMP3 converts raw WAV bytes to MP3 using ffmpeg.
+// ffmpeg reads from stdin (pipe:0) and writes to stdout (pipe:1).
+func wavToMP3(ctx context.Context, wav []byte) ([]byte, error) {
+	cmd := exec.CommandContext(ctx,
+		"ffmpeg",
+		"-hide_banner", "-loglevel", "error",
+		"-i", "pipe:0", // read WAV from stdin
+		"-f", "mp3", // output format
+		"-q:a", "2", // VBR quality ~190 kbps
+		"pipe:1", // write MP3 to stdout
+	)
+	cmd.Stdin = bytes.NewReader(wav)
+
+	var out, stderr bytes.Buffer
+	cmd.Stdout = &out
+	cmd.Stderr = &stderr
+
+	if err := cmd.Run(); err != nil {
+		return nil, fmt.Errorf("ffmpeg: %w (stderr: %s)", err, stderr.String())
+	}
+	return out.Bytes(), nil
+}