Read-aloud: natural neural voice via local Piper TTS

Replace the browser's robotic Web Speech API (espeak on Chromium) as the
primary read-aloud path with server-side Piper neural TTS, served by petal
and kept fully offline on millenia next to Ollama.

- internal/tts: proxy short passages to Piper, transcode WAV -> mp3/opus via
  ffmpeg, content-addressed disk cache (instant re-taps). Each Piper instance
  loads one voice, so language routes to its own endpoint:{voice}. Unknown
  language -> 404 so the client falls back to Web Speech. UTF-8-safe truncation
  for multibyte (Chinese) text.
- config: TTS_ENDPOINT / TTS_ENDPOINT_ZH / TTS_VOICE_EN / TTS_VOICE_ZH /
  TTS_CACHE_DIR / TTS_TIMEOUT / TTS_AUDIO_FORMAT. Route mounts only when
  TTS_ENDPOINT is set; otherwise unchanged behavior.
- web/audio/speech.ts: speak() hits /api/tts first, falls back to Web Speech on
  any failure; rapid-tap-safe via a request token. Call sites unchanged.
- deploy/: Piper user systemd units (EN :5005, ZH :5006), setup script, README.

English (en_US-amy-medium) and Chinese (zh_CN-huayan-medium) are both live.

Claude-Session: https://claude.ai/code/session_016Yr6jELuRc7hyzYLccQKZd

cmd/server/main.go

@@ -19,6 +19,7 @@ import (
 	"gitea.parodia.dev/drwily/petal/internal/lexicon"
 	"gitea.parodia.dev/drwily/petal/internal/llm"
 	"gitea.parodia.dev/drwily/petal/internal/suggestions"
+	"gitea.parodia.dev/drwily/petal/internal/tts"
 	"gitea.parodia.dev/drwily/petal/web"
 )
 
@@ -88,6 +89,14 @@ func main() {
 			log.Fatalf("image store: %v", err)
 		}
 		api.Mount("/images", imgHandler.Routes())
+
+		// Read-aloud: proxy short passages to a local Piper TTS server. Only
+		// mounted when TTS_ENDPOINT is configured; otherwise the frontend falls
+		// back to the browser's Web Speech API on its own.
+		if ttsHandler, ok := tts.New(cfg); ok {
+			api.Mount("/tts", ttsHandler.Routes())
+			log.Printf("read-aloud enabled (TTS endpoint=%s)", cfg.TTSEndpoint)
+		}
 	})
 
 	// Everything else: serve the embedded SPA (with index.html fallback for client routing).