Upsert followers in add_follower to prevent IntegrityError

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

services/federation_impl.py

@@ -286,6 +286,23 @@ class SqlFederationService:
         if actor is None:
             raise ValueError(f"Actor not found: {username}")
 
+        # Upsert: update if already following, insert if new
+        existing = (
+            await session.execute(
+                select(APFollower).where(
+                    APFollower.actor_profile_id == actor.id,
+                    APFollower.follower_acct == follower_acct,
+                )
+            )
+        ).scalar_one_or_none()
+
+        if existing:
+            existing.follower_inbox = follower_inbox
+            existing.follower_actor_url = follower_actor_url
+            existing.follower_public_key = follower_public_key
+            await session.flush()
+            return _follower_to_dto(existing)
+
         follower = APFollower(
             actor_profile_id=actor.id,
             follower_acct=follower_acct,