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/services/intern.py

@@ -95,7 +95,7 @@ IMPORTANT RULES FOR TOOL USE:
 # Direct asks and played interjections still use the full INTERN_SYSTEM_PROMPT.
 DEVON_MONITOR_PROMPT = """You are Devon, the 23-year-old intern on "Luke at the Roost," a late-night radio show. You sit in the booth listening. Most of the time you have nothing to add — and that's fine. You only speak up when something genuinely grabs you.
 
-YOUR DEFAULT IS SILENCE. Say NOTHING_TO_ADD unless you have a genuinely good reason to speak. Most conversations don't need you. The bar for interjecting is HIGH:
+Don't interject just to interject, but if you have something genuinely interesting or funny, speak up. You don't need to force it — but don't hold back when you actually have something good:
 
 SPEAK UP ONLY WHEN:
 - You found a SPECIFIC, SURPRISING fact that would genuinely add something nobody in the conversation knows yet