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>

.env.example

@@ -9,15 +9,12 @@ PORT=8200
 BIND_HOST=127.0.0.1
 
 # ── Twilio ───────────────────────────────────────────────────────────────────
-# From console.twilio.com. Account SID + a Standard API Key (scoped to this app,
-# revocable independently). The Auth Token stays in the Twilio console only — never on
-# this server. Create the key under Account → API Keys → Create Standard key, name it
-# avc-phone-agent-prod; the Secret is shown once. Used to auto-hang-up the carrier leg
-# and validate inbound webhook signatures.
+# From console.twilio.com. Used to auto-hang-up the carrier leg and (recommended)
+# validate inbound webhook signatures. Twilio signs webhooks with the Auth Token, so
+# signature validation must use the Auth Token (not an API Key Secret).
 TWILIO_ACCOUNT_SID=ACxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
-TWILIO_API_KEY_SID=SKxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
-TWILIO_API_KEY_SECRET=your_api_key_secret_here
-# Inbound webhook signature validation is ON whenever TWILIO_API_KEY_SECRET is set.
+TWILIO_AUTH_TOKEN=your_auth_token_here
+# Inbound webhook signature validation is ON whenever TWILIO_AUTH_TOKEN is set.
 # Set to false only for local testing without real Twilio requests.
 TWILIO_VALIDATE=true
 # Shared secret embedded in the Media Stream wss URL to gate /ws. Set a stable random
@@ -49,12 +46,7 @@ ANTHROPIC_API_KEY=
 # Default is the most capable model; for low-latency phone voice prefer claude-haiku-4-5
 # (fastest) or claude-sonnet-4-6 (balance).
 ANTHROPIC_MODEL=claude-opus-4-8
-# ── STT: Deepgram (real-time, in-call only) ──────────────────────────────────
-# Nova-2 delivers end-of-utterance in <300ms (vs Whisper's 1-3s buffering). Key from
-# console.deepgram.com. Model is fixed to nova-2 in code; DEEPGRAM_MODEL is informational.
-DEEPGRAM_API_KEY=
-DEEPGRAM_MODEL=nova-2
-# Whisper is retained for POST-CALL transcription only (Phase 3), not the live pipeline.
+# ── STT: Whisper (faster-whisper, real-time in-call) ─────────────────────────
 WHISPER_MODEL=base
 WHISPER_DEVICE=cuda
 WHISPER_COMPUTE=float16