Move auth server from federation to account

Account is now the OAuth authorization server with magic link login,
OAuth2 authorize endpoint, SSO logout, and session management.

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

blog/models/user.py

@@ -0,0 +1,4 @@
+# Re-export from canonical shared location
+from shared.models.user import User
+
+__all__ = ["User"]