feat(backend): real-time messaging via Socket.IO with persistent storage

[codebestia]

Summary

• Extended schema with conversations (dm/group enum), conversation_members, and messages tables; generated migration 0001_messaging.sql
• src/socket/messaging.ts — four Socket.IO event handlers:
  • create_conversation — creates a DM or group conversation, adds all members
  • join_room — subscribes the caller to a Socket.IO room after validating membership
  • send_message — persists message to DB, broadcasts new_message to all room members
  • message_history — fetches last 30 messages with cursor-based pagination (before message ID)
• All handlers guard against non-members and empty content
• GET /conversations REST route — returns all conversations + members for the authenticated user
• Registered messaging handlers in io.on('connection') alongside existing auth middleware