Fix duplicate AP posts + stable object IDs

- Stable object ID per source (Post#123 always gets the same id)
  instead of deriving from activity UUID
- Dedup Update activities (Ghost fires duplicate webhooks)
- Use setdefault for object id in delivery handler

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

events/handlers/ap_delivery_handler.py

@@ -38,7 +38,8 @@ def _build_activity_json(activity: APActivity, actor: ActorProfile, domain: str)
         obj.setdefault("type", "Tombstone")
     else:
         # Create/Update: full object with attribution
-        obj["id"] = object_id
+        # Prefer stable id from object_data (set by try_publish), fall back to activity-derived
+        obj.setdefault("id", object_id)
         obj.setdefault("type", activity.object_type)
         obj.setdefault("attributedTo", actor_url)
         obj.setdefault("published", activity.published.isoformat() if activity.published else None)