Add unpublish (Delete) support + improve object IDs

- on_post_unpublished handler sends Delete/Tombstone activity
- Create/Update objects use post URL as id (for Delete reference)
- Delete objects use Tombstone type

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

events/handlers/ap_delivery_handler.py

@@ -27,10 +27,16 @@ def _build_activity_json(activity: APActivity, actor: ActorProfile, domain: str)
     actor_url = f"https://{domain}/users/{username}"
 
     obj = dict(activity.object_data or {})
-    obj.setdefault("id", activity.activity_id + "/object")
-    obj.setdefault("type", activity.object_type)
-    obj.setdefault("attributedTo", actor_url)
-    obj.setdefault("published", activity.published.isoformat() if activity.published else None)
+
+    if activity.activity_type == "Delete":
+        # Delete: object is a Tombstone with just id + type
+        obj.setdefault("type", "Tombstone")
+    else:
+        # Create/Update: full object with attribution
+        obj.setdefault("id", obj.get("url") or (activity.activity_id + "/object"))
+        obj.setdefault("type", activity.object_type)
+        obj.setdefault("attributedTo", actor_url)
+        obj.setdefault("published", activity.published.isoformat() if activity.published else None)
 
     return {
         "@context": "https://www.w3.org/ns/activitystreams",