Fix per-app AP delivery, NULL uniqueness, and reverse discovery

- Delivery handler now signs/delivers using the per-app domain that
  matches the follower's subscription (not always federation domain)
- app_domain is NOT NULL with default 'federation' (sentinel replaces
  NULL to avoid uniqueness constraint edge case)
- Aggregate actor advertises per-app actors via alsoKnownAs
- Migration backfills existing NULL rows to 'federation'

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

services/federation_impl.py

@@ -339,7 +339,7 @@ class SqlFederationService:
         self, session: AsyncSession, username: str,
         follower_acct: str, follower_inbox: str, follower_actor_url: str,
         follower_public_key: str | None = None,
-        app_domain: str | None = None,
+        app_domain: str = "federation",
     ) -> APFollowerDTO:
         actor = (
             await session.execute(
@@ -350,16 +350,15 @@ class SqlFederationService:
             raise ValueError(f"Actor not found: {username}")
 
         # Upsert: update if already following this (actor, acct, app_domain)
-        q = select(APFollower).where(
-            APFollower.actor_profile_id == actor.id,
-            APFollower.follower_acct == follower_acct,
-        )
-        if app_domain is not None:
-            q = q.where(APFollower.app_domain == app_domain)
-        else:
-            q = q.where(APFollower.app_domain.is_(None))
-
-        existing = (await session.execute(q)).scalar_one_or_none()
+        existing = (
+            await session.execute(
+                select(APFollower).where(
+                    APFollower.actor_profile_id == actor.id,
+                    APFollower.follower_acct == follower_acct,
+                    APFollower.app_domain == app_domain,
+                )
+            )
+        ).scalar_one_or_none()
 
         if existing:
             existing.follower_inbox = follower_inbox
@@ -382,7 +381,7 @@ class SqlFederationService:
 
     async def remove_follower(
         self, session: AsyncSession, username: str, follower_acct: str,
-        app_domain: str | None = None,
+        app_domain: str = "federation",
     ) -> bool:
         actor = (
             await session.execute(
@@ -392,16 +391,13 @@ class SqlFederationService:
         if actor is None:
             return False
 
-        filters = [
-            APFollower.actor_profile_id == actor.id,
-            APFollower.follower_acct == follower_acct,
-        ]
-        if app_domain is not None:
-            filters.append(APFollower.app_domain == app_domain)
-        else:
-            filters.append(APFollower.app_domain.is_(None))
-
-        result = await session.execute(delete(APFollower).where(*filters))
+        result = await session.execute(
+            delete(APFollower).where(
+                APFollower.actor_profile_id == actor.id,
+                APFollower.follower_acct == follower_acct,
+                APFollower.app_domain == app_domain,
+            )
+        )
         return result.rowcount > 0
 
     async def get_followers_paginated(

services/stubs.py

@@ -235,10 +235,10 @@ class StubFederationService:
 
     async def add_follower(self, session, username, follower_acct, follower_inbox,
                            follower_actor_url, follower_public_key=None,
-                           app_domain=None):
+                           app_domain="federation"):
         raise RuntimeError("FederationService not available")
 
-    async def remove_follower(self, session, username, follower_acct, app_domain=None):
+    async def remove_follower(self, session, username, follower_acct, app_domain="federation"):
         return False
 
     async def get_or_fetch_remote_actor(self, session, actor_url):