fix: use correct model alias in AI_MODEL_TIERS standard tier

The dated model ID `claude-sonnet-4-6-20250514` was causing 502 errors.
Use the alias `claude-sonnet-4-6` which matches AI_MODEL_ANTHROPIC.

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

backend/app/core/config.py

@@ -87,7 +87,7 @@ class Settings(BaseSettings):
     # Model tier routing — maps action types to model tiers
     AI_MODEL_TIERS: dict[str, str] = {
         "fast": "claude-haiku-4-5-20251001",
-        "standard": "claude-sonnet-4-6-20250514",
+        "standard": "claude-sonnet-4-6",
     }
 
     ACTION_MODEL_MAP: dict[str, str] = {