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.

deploy/piper/Dockerfile

@@ -30,7 +30,7 @@ EXPOSE 5000
 # the honest check: it proves the model loaded, not just that a port is open.
 HEALTHCHECK --interval=60s --timeout=20s --start-period=180s --retries=3 \
   CMD python -c "import os,urllib.request,json; \
-urllib.request.urlopen(urllib.request.Request('http://127.0.0.1:'+os.environ['PIPER_PORT']+'/', \
+urllib.request.urlopen(urllib.request.Request('http://127.0.0.1:'+os.environ['PIPER_PORT']+'/synthesize', \
 data=json.dumps({'text':'ok','voice':os.environ['PIPER_VOICE']}).encode(), \
 headers={'Content-Type':'application/json'}), timeout=15).read(1)"