Add 'updated' timestamp to Update activity objects for Mastodon

Mastodon requires an updated field to process post edits.

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

events/handlers/ap_delivery_handler.py

@@ -42,6 +42,9 @@ def _build_activity_json(activity: APActivity, actor: ActorProfile, domain: str)
         obj.setdefault("published", activity.published.isoformat() if activity.published else None)
         obj.setdefault("to", ["https://www.w3.org/ns/activitystreams#Public"])
         obj.setdefault("cc", [f"{actor_url}/followers"])
+        if activity.activity_type == "Update":
+            from datetime import datetime, timezone
+            obj["updated"] = datetime.now(timezone.utc).isoformat()
 
     return {
         "@context": [