feat(tts): add Microsoft Edge TTS provider

[hagope]

Summary

Adds Microsoft Edge TTS as a fourth TTS provider (TTS_PROVIDER=edge). It uses Microsoft Edge's online Read Aloud service via WebSocket — no API key, account, or TTS_API_URL required; only outbound HTTPS/WebSocket access to speech.platform.bing.com.

Implementation ports the protocol from the Python edge-tts reference.

How it works

• DRM token (Sec-MS-GEC): SHA256 of Windows file-time ticks (rounded to 5 min) + trusted client token. Includes a single 403 retry that corrects clock skew from the server's Date header.
• SSML framing: prosody rate/volume/pitch, with 4096-byte UTF-8-safe chunking that never splits multi-byte characters or XML entities.
• WebSocket protocol: binary audio frames use a 2-byte big-endian header-length prefix (length value includes the trailing \r\n), so audio starts at offset 2 + headerLength; text frames signal turn.end per chunk.
• Output format: audio-24khz-48kbitrate-mono-mp3.

Changes

• New: src/app/services/edge-tts.ts + tests/app/services/edge-tts.test.ts
• Wire 'edge' provider into src/config.ts and src/app/services/tts-service.ts
• isTtsConfigured() returns true for edge (no credentials needed)
• Document provider in .env.example and PRODUCT.md
• Adds ws as a runtime dependency (Node 20+ target; the global WebSocket is only stable from Node 22+)

Configuration

TTS_PROVIDER=edge
TTS_VOICE=en-US-EmmaMultilingualNeural

Verification

• npm run build ✓
• npm run lint ✓ (zero warnings)
• npm test — all new tests pass (16 in edge-tts.test.ts + 4 edge cases in tts-service.test.ts)
  • Note: 1 pre-existing test failure in tests/config.test.ts (env-var leakage of TTS_MODEL) confirmed to also fail on main before this PR
• Verified end-to-end against the real Edge TTS service: produced a valid 2.5s MP3 (48 kbps, 24 kHz mono)