Make Piper's synthesis path configurable (TTS_PATH)

piper-tts 1.6.0 moved synthesis from POST / to POST /synthesize, with an
identical request body; the VPS sidecars run 1.6.0 and returned 405 to
every read-aloud request, while millenia's older server still expects /.
Rather than pinning both deployments to one Piper release, the path is
configuration -- default "/" keeps millenia working untouched, and the
compose stack sets /synthesize. The container healthcheck moves with it,
since it was probing the old route too.

internal/config/config.go

@@ -27,6 +27,11 @@ type Config struct {
 	TTSEndpointZH string // Piper instance serving the Chinese voice; empty = zh falls back to Web Speech
 	TTSVoiceEN    string // Piper voice id for English (e.g. en_US-amy-medium)
 	TTSVoiceZH    string // Piper voice id for Chinese (e.g. zh_CN-huayan-medium)
+	// TTSPath is the path Piper serves synthesis on. Piper moved it from "/" to
+	// "/synthesize" in 1.6.0 with an unchanged request body, so this is a
+	// version knob, not a feature: millenia's older server keeps the default,
+	// the containerised sidecars set "/synthesize".
+	TTSPath     string
 	TTSCacheDir string // on-disk store for synthesized clips (content-addressed)
 	TTSTimeout  time.Duration
 	TTSFormat   string // mp3 | opus | wav — mp3/opus transcode Piper's WAV via ffmpeg
@@ -56,6 +61,7 @@ func Load() *Config {
 		TTSEndpointZH: env("TTS_ENDPOINT_ZH", ""),
 		TTSVoiceEN:    env("TTS_VOICE_EN", "en_US-amy-medium"),
 		TTSVoiceZH:    env("TTS_VOICE_ZH", "zh_CN-huayan-medium"),
+		TTSPath:     env("TTS_PATH", "/"),
 		TTSCacheDir: env("TTS_CACHE_DIR", "./data/tts"),
 		TTSTimeout:  envDuration("TTS_TIMEOUT", 15*time.Second),
 		TTSFormat:   env("TTS_AUDIO_FORMAT", "mp3"),