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_test.go

@@ -36,12 +36,56 @@ func newHandler(t *testing.T, endpoint string) *Handler {
 	t.Helper()
 	return &Handler{
 		routes:   map[string]route{"en": {strings.TrimRight(endpoint, "/"), "en_US-amy-medium"}},
+		synthURI: synthPath("/"),
 		cacheDir: t.TempDir(),
 		format:   formats["wav"],
 		client:   http.DefaultClient,
 	}
 }
 
+// Piper 1.6.0 serves synthesis on /synthesize and 405s on /. The path is
+// configuration (TTS_PATH) so one Petal build talks to either server version;
+// this asserts the configured path is the one actually requested.
+func TestSynthUsesConfiguredPath(t *testing.T) {
+	var gotPath string
+	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
+		gotPath = r.URL.Path
+		if r.URL.Path != "/synthesize" {
+			w.WriteHeader(http.StatusMethodNotAllowed)
+			return
+		}
+		w.Header().Set("Content-Type", "audio/wav")
+		_, _ = w.Write([]byte("RIFF....fake-wav"))
+	}))
+	t.Cleanup(srv.Close)
+
+	h := newHandler(t, srv.URL)
+	h.synthURI = synthPath("/synthesize")
+
+	if rr := post(t, h, "hello there", "en-US"); rr.Code != http.StatusOK {
+		t.Fatalf("status = %d, want 200 (piper saw path %q)", rr.Code, gotPath)
+	}
+	if gotPath != "/synthesize" {
+		t.Errorf("piper path = %q, want /synthesize", gotPath)
+	}
+}
+
+func TestSynthPathNormalisation(t *testing.T) {
+	cases := map[string]string{
+		"":             "/",
+		"/":            "/",
+		"synthesize":   "/synthesize",
+		"/synthesize":  "/synthesize",
+		"/synthesize/": "/synthesize",
+		"  /v1/tts  ":  "/v1/tts",
+	}
+	for in, want := range cases {
+		if got := synthPath(in); got != want {
+			t.Errorf("synthPath(%q) = %q, want %q", in, got, want)
+		}
+	}
+}
+
 func post(t *testing.T, h *Handler, text, lang string) *httptest.ResponseRecorder {
 	t.Helper()
 	b, _ := json.Marshal(synthRequest{Text: text, Lang: lang})