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>

alembic/versions/t0r8n4o6p7_add_app_domain_to_ap_followers.py

@@ -13,10 +13,18 @@ depends_on = None
 
 
 def upgrade():
+    # Add column as nullable first so we can backfill
     op.add_column(
         "ap_followers",
         sa.Column("app_domain", sa.String(64), nullable=True),
     )
+    # Backfill existing rows: all current followers are aggregate
+    op.execute("UPDATE ap_followers SET app_domain = 'federation' WHERE app_domain IS NULL")
+    # Now make it NOT NULL with a default
+    op.alter_column(
+        "ap_followers", "app_domain",
+        nullable=False, server_default="federation",
+    )
     # Replace old unique constraint with one that includes app_domain
     op.drop_constraint("uq_follower_acct", "ap_followers", type_="unique")
     op.create_unique_constraint(
@@ -39,4 +47,5 @@ def downgrade():
         "ap_followers",
         ["actor_profile_id", "follower_acct"],
     )
+    op.alter_column("ap_followers", "app_domain", nullable=True, server_default=None)
     op.drop_column("ap_followers", "app_domain")