Fix activity ID domain mismatch in per-app delivery

Mastodon requires the activity ID host to match the actor host.
The stored activity_id uses federation.rose-ash.com but per-app
delivery sends actor as blog.rose-ash.com etc.  Rewrite the
activity ID host to match the delivery domain.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

events/handlers/ap_delivery_handler.py

@@ -44,9 +44,15 @@ def _build_activity_json(activity: APActivity, actor: ActorProfile, domain: str)
 
     obj = dict(activity.object_data or {})
 
-    # Object id MUST be on the actor's domain (Mastodon origin check).
-    # The post URL (e.g. blog.rose-ash.com/slug/) goes in "url" only.
-    object_id = activity.activity_id + "/object"
+    # Rewrite activity ID to match the delivery domain so Mastodon's
+    # origin check passes (activity ID host must equal actor host).
+    # The stored activity_id uses the federation domain; we replace
+    # just the host portion for per-app delivery.
+    import re
+    activity_id = re.sub(
+        r"^https://[^/]+/", f"https://{domain}/", activity.activity_id,
+    )
+    object_id = activity_id + "/object"
 
     if activity.activity_type == "Delete":
         obj.setdefault("id", object_id)
@@ -67,7 +73,7 @@ def _build_activity_json(activity: APActivity, actor: ActorProfile, domain: str)
             "https://www.w3.org/ns/activitystreams",
             "https://w3id.org/security/v1",
         ],
-        "id": activity.activity_id,
+        "id": activity_id,
         "type": activity.activity_type,
         "actor": actor_url,
         "published": activity.published.isoformat() if activity.published else None,