Inline federation publication + fix AP delivery

- Replace async federation_handlers with inline try_publish() at write sites
- Fix ap_delivery_handler: urlparse for signature path/host, @context array
  with security vocab, Delete/Tombstone object handling
- Fix federation_impl: @context array for IPFS, .limit(1) + upsert follower

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

events/handlers/ap_delivery_handler.py

@@ -39,7 +39,10 @@ def _build_activity_json(activity: APActivity, actor: ActorProfile, domain: str)
         obj.setdefault("published", activity.published.isoformat() if activity.published else None)
 
     return {
-        "@context": "https://www.w3.org/ns/activitystreams",
+        "@context": [
+            "https://www.w3.org/ns/activitystreams",
+            "https://w3id.org/security/v1",
+        ],
         "id": activity.activity_id,
         "type": activity.activity_type,
         "actor": actor_url,