fix: correct Anthropic model ID to full dated version

claude-haiku-4-5 is not a valid model alias — Anthropic requires the
full dated model ID claude-haiku-4-5-20251001.

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

backend/app/core/config.py

@@ -74,7 +74,7 @@ class Settings(BaseSettings):
 
     # AI Flow Builder
     ANTHROPIC_API_KEY: Optional[str] = None
-    AI_MODEL: str = "claude-haiku-4-5"
+    AI_MODEL: str = "claude-haiku-4-5-20251001"
     AI_CONVERSATION_TTL_HOURS: int = 24
     AI_MAX_CALLS_PER_FLOW: int = 10
     AI_REQUEST_TIMEOUT_SECONDS: int = 45