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/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):