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>

contracts/dtos.py

@@ -176,7 +176,7 @@ class APFollowerDTO:
     follower_inbox: str
     follower_actor_url: str
     created_at: datetime | None = None
-    app_domain: str | None = None
+    app_domain: str = "federation"
 
 
 @dataclass(frozen=True, slots=True)

contracts/protocols.py

@@ -249,12 +249,12 @@ class FederationService(Protocol):
         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: ...
 
     async def remove_follower(
         self, session: AsyncSession, username: str, follower_acct: str,
-        app_domain: str | None = None,
+        app_domain: str = "federation",
     ) -> bool: ...
 
     # -- Remote actors --------------------------------------------------------