fix(config): allow extra env vars and make OAuth fields optional

Pre-existing issues fixed:
- Add extra='ignore' to Settings model_config for backward compatibility
  with legacy env vars (GEMINI_API_KEY, OPENAI_API_KEY)
- Add default empty values to GITHUB_CLIENT_ID and GITHUB_CLIENT_SECRET
  for testing/development environments

These changes allow tests to run without requiring full production config.

Author: ComBba

backend/app/core/config.py

@@ -20,6 +20,7 @@ class Settings(BaseSettings):
         env_file=".env",
         env_file_encoding="utf-8",
         case_sensitive=True,
+        extra="ignore",
     )
 
     APP_NAME: str = "Somm.dev API"
@@ -31,9 +32,9 @@ class Settings(BaseSettings):
     JWT_ALGORITHM: str = "HS256"
     JWT_EXPIRATION_DAYS: int = 7
 
-    # GitHub OAuth
-    GITHUB_CLIENT_ID: str
-    GITHUB_CLIENT_SECRET: str
+    # GitHub OAuth (required for production, optional for testing)
+    GITHUB_CLIENT_ID: str = ""
+    GITHUB_CLIENT_SECRET: str = ""
 
     # URLs
     FRONTEND_URL: str = "https://www.somm.dev"