Grok 4 routing, guardrails, pricing fix, strip silence improvements

- Route caller_dialog, devon_ask, background_gen to x-ai/grok-4
- Add Grok-4 to OPENROUTER_MODELS and OPENROUTER_PRICING
- Add Grok-specific banned phrases (I hear you, fair enough, that's wild, etc.)
- Add background gen guardrails for Grok (no active violence, no real public figures)
- Soften theme prompt hot-take language for organic connections
- Tighten Devon flirting guardrail (awkward not crude)
- Fix Devon "first day" contradiction on line 36
- Strip silence: preserve music intro, fix ad normalization (direct WAV reading)
- Strip silence: loop range starts 0.5s before audible music

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

backend/services/cost_tracker.py

@@ -35,6 +35,7 @@ OPENROUTER_PRICING = {
     "anthropic/claude-sonnet-4-5":      {"prompt": 3.00,  "completion": 15.00},
     "anthropic/claude-haiku-4.5":       {"prompt": 0.80,  "completion": 4.00},
     "anthropic/claude-3-haiku":         {"prompt": 0.25,  "completion": 1.25},
+    "x-ai/grok-4":                     {"prompt": 3.00,  "completion": 15.00},
     "x-ai/grok-4-fast":                {"prompt": 5.00,  "completion": 15.00},
     "minimax/minimax-m2-her":           {"prompt": 0.50,  "completion": 1.50},
     "mistralai/mistral-small-creative": {"prompt": 0.20,  "completion": 0.60},