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>

infrastructure/activitypub.py

@@ -64,8 +64,8 @@ def create_activitypub_blueprint(app_name: str) -> Blueprint:
     domain = _ap_domain(app_name)
     fed_domain = _federation_domain()
     aggregate = _is_aggregate(app_name)
-    # For per-app follows, store app_domain; for federation aggregate, NULL
-    follower_app_domain: str | None = None if aggregate else app_name
+    # For per-app follows, store app_domain; for federation, "federation"
+    follower_app_domain: str = app_name
     # For per-app outboxes, filter by origin_app; for federation, show all
     outbox_origin_app: str | None = None if aggregate else app_name
 
@@ -201,8 +201,16 @@ def create_activitypub_blueprint(app_name: str) -> Blueprint:
                 "url": actor_url,
             }
 
-            # Per-app actors link back to the aggregate federation actor
-            if not aggregate and domain != fed_domain:
+            if aggregate:
+                # Aggregate actor advertises all per-app actors
+                also_known = [
+                    f"https://{_ap_domain(a)}/users/{username}"
+                    for a in AP_APPS if a != "federation"
+                ]
+                if also_known:
+                    actor_json["alsoKnownAs"] = also_known
+            else:
+                # Per-app actors link back to the aggregate federation actor
                 actor_json["alsoKnownAs"] = [
                     f"https://{fed_domain}/users/{username}",
                 ]