feat: replace MeloTTS with Coqui XTTS-v2 natural Korean voice

MeloTTS's single Korean speaker sounded non-native ("foreign accent"). Swap it
for Coqui XTTS-v2 with the built-in female studio speaker "Ana Florence"
(language ko), the natural voice used in earlier local runs.

- bridge/xtts_worker.py: new warm HTTP worker (own /opt/xtts venv), same
  /synth + /health contract and PCM16 output as the old melo worker
- docker/setup-xtts.sh: builds the venv with cu128 torch (Blackwell) + Coqui
  TTS and bakes the XTTS-v2 model offline. Pins transformers>=4.57,<5 (5.x
  removed isin_mps_friendly, breaking XTTS) and installs the [codec] extra
  (torch>=2.9 needs torchcodec) — both verified by a real host synth
- Dockerfile: replace the melo build layer with the xtts layer
- supervisord.conf: melo-worker -> xtts-worker, env passthrough for
  XTTS_DEVICE/SPEAKER/LANGUAGE (always set via compose defaults)
- bridge/server.py: default TTS_ENGINE=xtts, route to the xtts worker, generic
  worker-synth helper, neural-only fallback flag (XTTS_FALLBACK_PIPER)
- settings UI: engine dropdown xtts/piper, drop the dead melo_speed field, fix
  the supervisorctl restart target to xtts-worker
- compose/.env.example/README: XTTS_* vars, speaker/language knobs, remove melo
- remove bridge/melo_worker.py and docker/setup-melo.sh
- tests: xtts treated as multilingual (not English-only)

Verified on host: coqui-tts loads XTTS-v2 and synthesises Korean as
"Ana Florence" to a 16-bit mono 24kHz WAV.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

tests/test_system_prompt.py

@@ -101,6 +101,14 @@ class TestReplyLanguageDirective:
         # user's own language, so no directive.
         assert reply_language_directive(None, "melo") is None
 
+    def test_xtts_is_multilingual(self):
+        # XTTS-v2 (the Korean voice) is not English-only: no lock → free, and a
+        # lock is honoured (not overridden to English).
+        assert reply_language_directive(None, "xtts") is None
+        directive = reply_language_directive("Korean", "xtts")
+        assert directive is not None and "Korean" in directive
+        assert directive != ENGLISH_ONLY_DIRECTIVE
+
     def test_unknown_tts_defaults_to_english_only(self):
         # Preserves the original getattr(cfg, 'tts_engine', 'piper') default:
         # an unknown/missing engine is treated conservatively as English-only.