feat: use Edge TTS (Korean Hyunsu voice @ +45%) as the default voice

The user chose Microsoft Edge TTS, voice ko-KR-HyunsuMultilingualNeural at rate
+45% (~1.45x), as the natural Korean voice. Wire it into the bridge and make it
the default engine.

- bridge/server.py: _edge_synthesize() calls edge-tts and transcodes the MP3 to
  PCM16 mono WAV with the system ffmpeg (temp file for a correct header);
  TTS_ENGINE default -> edge; EDGE_TTS_VOICE / EDGE_TTS_RATE env-driven
- requirements-bridge.txt: add edge-tts (lightweight; httpx)
- compose/.env.example/README: TTS_ENGINE=edge + EDGE_TTS_* knobs; note the
  online/privacy trade-off (reply text is sent to Microsoft, needs internet)
- drop the now-unused MeloTTS build layer (Dockerfile) and melo-worker
  (supervisord) — edge synthesises in-process, no model/worker baked, slimmer
  and faster image; settings UI engine list -> edge/piper, restart only bridge

Verified on host: edge-tts -> ffmpeg yields a valid 16-bit mono 24kHz WAV;
envsubst renders tts_engine=edge; docker build --check + 26 tests pass.

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

Dockerfile

@@ -65,20 +65,14 @@ RUN ls -d /opt/venv/lib/python*/site-packages/nvidia/cublas/lib \
         > /etc/ld.so.conf.d/nvidia-cu12.conf 2>/dev/null \
     && /sbin/ldconfig || true
 
-# --- MeloTTS Korean voice (separate /opt/melo py3.11 venv; see setup-melo.sh).
-#     Heavy layer (torch CPU + transformers + MeCab); placed before the app
-#     COPY so it stays cached across source-only changes. ---
-COPY docker/setup-melo.sh /app/docker/setup-melo.sh
-# Strip CR before running: a Windows checkout (autocrlf) yields CRLF, which makes
-# bash read line 18 as `set -euxo pipefail\r` and abort with
-# "set: pipefail: invalid option name". .gitattributes pins *.sh to LF, but this
-# keeps the build working even on a not-yet-renormalised working tree.
-RUN sed -i 's/\r$//' /app/docker/setup-melo.sh && bash /app/docker/setup-melo.sh
+# --- Korean voice: Microsoft Edge TTS (online neural). No model is baked — the
+#     `edge-tts` pip package (in requirements-bridge.txt) calls the MS service at
+#     runtime and the bridge transcodes the MP3 to PCM16 with ffmpeg. No heavy
+#     TTS build layer is needed. ---
 
 # --- Human input + window management for the on-screen Chrome control tool.
-#     Placed AFTER the heavy melo layer so it doesn't bust that cache. xdotool
-#     injects real X pointer/keyboard events (visible cursor, char-by-char
-#     typing) into the broadcast; wmctrl lists/moves windows. ---
+#     xdotool injects real X pointer/keyboard events (visible cursor,
+#     char-by-char typing) into the broadcast; wmctrl lists/moves windows. ---
 RUN apt-get update && apt-get install -y --no-install-recommends \
       xdotool wmctrl \
     && rm -rf /var/lib/apt/lists/*