Remove auth blueprint, federation is now an OAuth client

Auth server responsibilities moved to account app.
Federation uses the shared OAuth client blueprint via factory.

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

bp/__init__.py

@@ -1,5 +1,4 @@
 from .wellknown.routes import register as register_wellknown_bp
 from .actors.routes import register as register_actors_bp
 from .identity.routes import register as register_identity_bp
-from .auth.routes import register as register_auth_bp
 from .social.routes import register as register_social_bp