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.

docker-compose.yml

@@ -42,6 +42,9 @@ services:
       # separate containers; the handler maps language → instance from config.
       TTS_ENDPOINT: http://piper-en:5000
       TTS_ENDPOINT_ZH: http://piper-zh:5000
+      # The sidecars run piper-tts 1.6.0, which serves synthesis on
+      # /synthesize; millenia's older server keeps the default "/".
+      TTS_PATH: /synthesize
       # The companion's bedtime nag and night mode read the local clock.
       TZ: ${TZ:-Europe/Lisbon}
     volumes: