Implements chat functionality by defining enums and database schema for contacts, chats, participants, and messages in `shared/schema.ts`. Replit-Commit-Author: Agent Replit-Commit-Session-Id: c5f0c281-8dd8-4846-b452-4a07bcd21062 Replit-Commit-Screenshot-Url: https://storage.googleapis.com/screenshot-production-us-central1/9777c70b-fc38-4831-8d6b-78dfffe041b0/75403342-40aa-4aa9-94d7-a6ce1fe4679a.jpg