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>

services/federation_impl.py

@@ -163,7 +163,10 @@ class SqlFederationService:
             from shared.utils.ipfs_client import add_json, is_available
             if await is_available():
                 activity_json = {
-                    "@context": "https://www.w3.org/ns/activitystreams",
+                    "@context": [
+                        "https://www.w3.org/ns/activitystreams",
+                        "https://w3id.org/security/v1",
+                    ],
                     "id": activity_uri,
                     "type": activity_type,
                     "actor": f"https://{domain}/users/{username}",