Fix OAuth authorize URL: include /auth prefix

The federation auth blueprint is mounted at /auth, so the authorize
endpoint is /auth/oauth/authorize, not /oauth/authorize.

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

infrastructure/oauth.py

@@ -42,7 +42,7 @@ def create_oauth_blueprint(app_name: str) -> Blueprint:
 
         redirect_uri = app_url(app_name, "/auth/callback")
         authorize_url = federation_url(
-            f"/oauth/authorize?client_id={app_name}"
+            f"/auth/oauth/authorize?client_id={app_name}"
             f"&redirect_uri={redirect_uri}"
             f"&state={state}"
         )