feat: add Gemini Flash config vars to Settings

Adds AI_PROVIDER, GOOGLE_AI_API_KEY, AI_MODEL_GEMINI, and
AI_MODEL_ANTHROPIC config vars. Updates ai_enabled to check
either provider key.

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

backend/app/core/config.py

@@ -78,11 +78,16 @@ class Settings(BaseSettings):
     AI_CONVERSATION_TTL_HOURS: int = 24
     AI_MAX_CALLS_PER_FLOW: int = 10
     AI_REQUEST_TIMEOUT_SECONDS: int = 45
+    # AI Provider selection
+    AI_PROVIDER: str = "gemini"  # "gemini" or "anthropic"
+    GOOGLE_AI_API_KEY: Optional[str] = None
+    AI_MODEL_GEMINI: str = "gemini-2.5-flash"
+    AI_MODEL_ANTHROPIC: str = "claude-haiku-4-5-20251001"
 
     @property
     def ai_enabled(self) -> bool:
-        """Check if AI Flow Builder is configured."""
-        return self.ANTHROPIC_API_KEY is not None
+        """Check if any AI provider is configured."""
+        return self.ANTHROPIC_API_KEY is not None or self.GOOGLE_AI_API_KEY is not None
 
     # Deployment – auto-seed test data on PR environments
     SEED_ON_DEPLOY: bool = False