Revert Phase 1 STT/auth swaps: stay on Whisper + Twilio Auth Token

Deepgram and the Twilio Standard API Key were reverted per decision:
- bot.py: restore HintedWhisperSTTService (faster-whisper hotwords), default
  model medium; remove DeepgramSTTService import + DEEPGRAM_API_KEY.
- server.py: restore TWILIO_AUTH_TOKEN for X-Twilio-Signature validation and
  the serializer auto-hang-up. Twilio signs webhooks with the Auth Token, so
  an API Key Secret cannot validate signatures.
- .env.example: back to TWILIO_AUTH_TOKEN + Whisper STT vars.
- .gitignore: ignore runtime *.log (avc_run.log).

OLLAMA_MODEL stays activeblue-avc:latest (the existing pulled tag).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

.gitignore

@@ -1,6 +1,10 @@
 # Secrets — never commit
 .env
 
+# Runtime logs
+avc_run.log
+*.log
+
 # Recordings (local only, may contain PHI)
 recordings/