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/config.py

@@ -37,10 +37,10 @@ class Settings(BaseSettings):
     # Categories: caller_dialog, devon_monitor, devon_ask, background_gen,
     #             call_summary, news_summary, topic_gen, unknown
     category_models: dict = {
-        "caller_dialog": "x-ai/grok-4-fast",                   # testing edgier dialog — revert to anthropic/claude-sonnet-4-5
-        "devon_ask": "google/gemini-2.5-flash",               # Devon direct questions
-        "devon_monitor": "google/gemini-2.5-flash",           # Devon polling — biggest cost saver
-        "background_gen": "google/gemini-2.5-flash",          # JSON caller backgrounds
+        "caller_dialog": "x-ai/grok-4",                         # full Grok 4 — edgier dialog, latency OK (gaps cut in post)
+        "devon_ask": "x-ai/grok-4",                             # Devon should match the show's edgy energy
+        "devon_monitor": "google/gemini-2.5-flash",           # Devon polling — just decisions, keep cheap
+        "background_gen": "x-ai/grok-4",                      # wilder, more specific caller backgrounds
         "call_summary": "google/gemini-2.5-flash",            # post-call summaries
         "news_summary": "google/gemini-2.5-flash",            # news digests
         "topic_gen": "google/gemini-2.5-flash",               # topic generation