feat(auth): add Google OAuth callback with oauth_identities linking

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend/app/core/config.py

@@ -194,6 +194,13 @@ class Settings(BaseSettings):
         """Check if ConnectWise integration is configured."""
         return self.CW_CLIENT_ID is not None
 
+    # OAuth providers (self-serve signup)
+    GOOGLE_CLIENT_ID: Optional[str] = None
+    GOOGLE_CLIENT_SECRET: Optional[str] = None
+    MS_CLIENT_ID: Optional[str] = None
+    MS_CLIENT_SECRET: Optional[str] = None
+    OAUTH_REDIRECT_BASE: str = "http://localhost:5173"
+
     # Monitoring
     SENTRY_DSN: Optional[str] = None