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>

models/federation.py

@@ -129,8 +129,9 @@ class APActivity(Base):
 class APFollower(Base):
     """A remote follower of a local actor.
 
-    ``app_domain`` scopes the follow to a specific app (e.g. "blog").
-    NULL means the follower subscribes to the aggregate (all activities).
+    ``app_domain`` scopes the follow to a specific app (e.g. "blog",
+    "market", "events").  "federation" means the aggregate — the
+    follower subscribes to all activities.
     """
     __tablename__ = "ap_followers"
 
@@ -142,7 +143,9 @@ class APFollower(Base):
     follower_inbox: Mapped[str] = mapped_column(String(512), nullable=False)
     follower_actor_url: Mapped[str] = mapped_column(String(512), nullable=False)
     follower_public_key: Mapped[str | None] = mapped_column(Text, nullable=True)
-    app_domain: Mapped[str | None] = mapped_column(String(64), nullable=True)
+    app_domain: Mapped[str] = mapped_column(
+        String(64), nullable=False, default="federation", server_default="federation",
+    )
     created_at: Mapped[datetime] = mapped_column(
         DateTime(timezone=True), nullable=False, server_default=func.now(),
     )