feat: restrict community creation to instance DID

Only did:web:coves.social can now create communities in production.

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude <[e-mail]>

docker-compose.prod.yml

···
 
       # Cursor encryption for pagination
       CURSOR_SECRET: ${CURSOR_SECRET}
+
+      # Restrict community creation to instance DID only
+      COMMUNITY_CREATORS: did:web:coves.social
     networks:
       - coves-internal
     depends_on: