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/tts/handler.go

@@ -56,6 +56,25 @@ var formats = map[string]audioFormat{
 		ffmpegArgs: []string{"-f", "ogg", "-c:a", "libopus", "-b:a", "32k", "-ac", "1"}},
 }
 
+// synthPath normalises TTS_PATH into a leading-slash path with no trailing
+// slash, so it concatenates cleanly onto a route's endpoint.
+//
+// Piper moved synthesis from `POST /` to `POST /synthesize` in 1.6.0, and the
+// request body is identical either side of that change. Rather than pinning
+// every deployment to one Piper release, the path is configuration: millenia
+// keeps the default `/` its installed server expects, and the containerised
+// 1.6.0 sidecars on the VPS set `/synthesize`.
+func synthPath(p string) string {
+	p = strings.TrimSpace(p)
+	if p == "" || p == "/" {
+		return "/"
+	}
+	if !strings.HasPrefix(p, "/") {
+		p = "/" + p
+	}
+	return strings.TrimRight(p, "/")
+}
+
 // route is the Piper instance and voice id serving one language. Each Piper
 // HTTP server loads exactly one model, so distinct languages mean distinct
 // endpoints (e.g. English on :5005, Chinese on :5006).
@@ -67,6 +86,7 @@ type route struct {
 // Handler proxies synthesis to Piper and caches the result on disk.
 type Handler struct {
 	routes   map[string]route // base language (e.g. "en", "zh") -> Piper instance
+	synthURI string           // path Piper serves synthesis on (see TTSPath)
 	cacheDir string
 	format   audioFormat
 	client   *http.Client
@@ -105,6 +125,7 @@ func New(cfg *config.Config) (*Handler, bool) {
 
 	return &Handler{
 		routes:   routes,
+		synthURI: synthPath(cfg.TTSPath),
 		cacheDir: cfg.TTSCacheDir,
 		format:   format,
 		client:   &http.Client{Timeout: cfg.TTSTimeout},
@@ -207,7 +228,7 @@ func (h *Handler) synthesize(ctx context.Context, rt route, text string) ([]byte
 		"voice":        rt.voice,
 		"length_scale": lengthScale,
 	})
-	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, rt.endpoint+"/", bytes.NewReader(body))
+	httpReq, err := http.NewRequestWithContext(ctx, http.MethodPost, rt.endpoint+h.synthURI, bytes.NewReader(body))
 	if err != nil {
 		return nil, err
 	}