feat(voice): Pas 2 — install voice deps, vendor discord-ext-voice-recv, setup assets

Foundation pentru Discord voice-to-voice pipeline.

- requirements.txt: faster-whisper, silero-vad, num2words, numpy, PyNaCl
- vendor/discord-ext-voice-recv/: vendored la commit ac04ea7b09 (bump version
  0.5.3a) — Discord voice protocol fragil, upstream hobby fork. Adapter layer
  in src/voice/_discord_voice_adapter.py izolează churn (swap la py-cord =
  doar acel fișier rescris). VENDOR_INFO.md documentează update procedure.
- tools/voice_setup.py: idempotent setup script — libopus check, ffmpeg
  check, Supertonic reachable, faster-whisper/silero-vad warm, assets
  generation. Exit 0 = green, 1 = needs human (currently libopus missing
  needs `sudo apt install -y libopus0`).
- assets/voice/: thinking.wav (filler "Stai puțin să-mi adun gândurile",
  ~2.8s), mhm.wav (listener noise), beep_200ms.wav (wake-up tone 880Hz).
- src/voice/__init__.py: package stub.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

vendor/discord-ext-voice-recv/VENDOR_INFO.md

@@ -0,0 +1,22 @@
+# Vendored: discord-ext-voice-recv
+
+**Upstream:** https://github.com/imayhaveborkedit/discord-ext-voice-recv
+**Pinned commit:** `ac04ea7b0941112e83767cf1c1469b408fa06748` (bump version 0.5.3a)
+**Vendored at:** 2026-05-27
+**Reason:** Discord voice protocol is fragile, upstream is hobby fork. Adapter
+layer in `src/voice/_discord_voice_adapter.py` isolates upstream churn — if this
+package breaks, swap to py-cord by rewriting only that file.
+
+## Update procedure
+
+```bash
+cd vendor/discord-ext-voice-recv
+git fetch origin master
+git log HEAD..origin/master --oneline  # review what changed
+git checkout <new-commit>
+cd ../..
+source .venv/bin/activate && pip install -e vendor/discord-ext-voice-recv --force-reinstall
+pytest tests/test_voice_adapter_contract.py -v  # MUST PASS — contract guard
+```
+
+Update this file's `Pinned commit` after a successful upgrade.