Fix AP Delete: Tombstone id must match original Create object id

Mastodon ignored Delete activities because the Tombstone id was the
post URL, not the object id from the original Create activity. Now
looks up the existing Create activity and uses its object id.

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

services/federation_publish.py

@@ -53,6 +53,10 @@ async def try_publish(
     elif activity_type == "Delete":
         return  # never published, nothing to delete
 
+    # Delete must reference the same object id Mastodon received in Create
+    if activity_type == "Delete" and existing:
+        object_data["id"] = existing.activity_id + "/object"
+
     try:
         await services.federation.publish_activity(
             session,