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>

docker-compose.yml

@@ -66,13 +66,15 @@ services:
       WHISPER_MODEL: ${WHISPER_MODEL:-medium}
       WHISPER_DEVICE: ${WHISPER_DEVICE:-cuda}
       WHISPER_COMPUTE_TYPE: ${WHISPER_COMPUTE_TYPE:-float16}
-      # MeloTTS on the GPU (cu128 torch baked by docker/setup-melo.sh). CPU synth
-      # serialised under load and pushed TTS to 7-8s; GPU does ~0.3s/sentence.
-      MELO_DEVICE: ${MELO_DEVICE:-cuda}
-      # Speaking rate for MeloTTS. Set here (with a default) so supervisord's
-      # %(ENV_MELO_SPEED)s passthrough always resolves and an .env override
-      # actually reaches the melo-worker. Lower it (e.g. 1.1) for a calmer pace.
-      MELO_SPEED: ${MELO_SPEED:-1.5}
+      # Coqui XTTS-v2 (natural female Korean voice, replaces MeloTTS) on the GPU
+      # (cu128 torch baked by docker/setup-xtts.sh). Set here WITH DEFAULTS so
+      # supervisord's %(ENV_XTTS_*)s passthrough always resolves and an .env
+      # override actually reaches the xtts-worker.
+      XTTS_DEVICE: ${XTTS_DEVICE:-cuda}
+      # Built-in studio speaker (female). Other options include "Daisy Studious",
+      # "Sofia Hellen", "Alma María", etc. — any XTTS-v2 studio speaker name.
+      XTTS_SPEAKER: ${XTTS_SPEAKER:-Ana Florence}
+      XTTS_LANGUAGE: ${XTTS_LANGUAGE:-ko}
       # Optional single-language lock for replies (empty = user's own language).
       OUTPUT_LANGUAGE: ${OUTPUT_LANGUAGE:-ko}
       # Drop the pre-loop planner LLM call to cut voice-reply latency on small