introduce separate SIMPLE_MODEL config for lightweight tasks

Add a distinct simple_model field alongside ai_model so operators can
route cheap, simple tasks (e.g. email-label translation) to a low-cost
model while keeping the strong model for parent conversations. The two
models can be from different providers (e.g. Gemini + Haiku). If
SIMPLE_MODEL is unset, falls back to AI_MODEL with a warning.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

chat_app.py

@@ -58,6 +58,7 @@ _notifier = AdminNotifier(
     indoor_email=_config.admin_email_indoor,
     outdoor_email=_config.admin_email_outdoor,
     cc_emails=[e.strip() for e in _config.admin_email_cc.split(",") if e.strip()],
+    model=_config.simple_model,
 )
 
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