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>

blog/models/ghost_membership_entities.py

@@ -0,0 +1,12 @@
+# Re-export from canonical shared location
+from shared.models.ghost_membership_entities import (
+    GhostLabel, UserLabel,
+    GhostNewsletter, UserNewsletter,
+    GhostTier, GhostSubscription,
+)
+
+__all__ = [
+    "GhostLabel", "UserLabel",
+    "GhostNewsletter", "UserNewsletter",
+    "GhostTier", "GhostSubscription",
+]