feat: multi-provider OAuth callback + GitHub config + login buttons

- Callback now supports any provider (Gitea/GitHub) via providers.get_provider()
- OAuth tokens stored in user_oauth_tokens table per user + provider
- Login page shows both Gitea and GitHub OAuth buttons
- config.py: github_client_id + github_client_secret
- Auth flow no longer depends on gitea_oauth import — fully abstracted
- Fallback admin user now sets is_active=1 and admin@localhost email

app/config.py

@@ -17,6 +17,10 @@ class Settings(BaseSettings):
     gitea_oauth_client_secret: str = ""
     gitea_webhook_secret: str = ""
 
+    # GitHub
+    github_client_id: str = ""
+    github_client_secret: str = ""
+
     # OAuth2
     oauth_redirect_uri: str = "http://localhost:8080/auth/callback"