Fix tonight's show issues: Whisper bias, boring callers, Devon, short responses

- Remove caller names from Whisper hint (was corrupting transcriptions)
- Background gen switched to Claude Sonnet 4.6 (cheap models = thin backgrounds)
- "WHAT MAKES A GOOD CALLER" rewritten with concrete examples
- Grok guardrails loosened (were cutting too much edge)
- Response length guidance added to caller prompt
- Retry under-20-word responses once for more detail
- Devon monitor softened from "default silence" to balanced
- Ban stalling phrases: "where was I", "as I was saying", etc.

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

backend/config.py

@@ -39,7 +39,7 @@ class Settings(BaseSettings):
         "caller_dialog": "x-ai/grok-4.1-fast",               # fallback if style_matched disabled ($0.20/$0.50)
         "devon_ask": "x-ai/grok-4.1-fast",                   # Devon matches show energy, cheap ($0.20/$0.50)
         "devon_monitor": "google/gemini-2.5-flash",          # just yes/no decisions, keep cheap ($0.15/$0.60)
-        "background_gen": "x-ai/grok-4.1-fast",              # wilder caller backgrounds ($0.20/$0.50)
+        "background_gen": "anthropic/claude-sonnet-4.6",       # backgrounds drive the whole call — worth the quality ($3/$15, ~$0.30/show)
         "call_summary": "google/gemini-2.5-flash",           # post-call, no personality needed ($0.15/$0.60)
         "news_summary": "google/gemini-2.5-flash",           # just digesting headlines ($0.15/$0.60)
         "topic_gen": "google/gemini-2.5-flash",              # structured output ($0.15/$0.60)