core: implement real-time events via Supabase Realtime

[EmeditWeb]

Problem

The web app and mobile app must poll the API to
get updates. No real-time notification exists.

What To Build

1. Enable Supabase Realtime on key tables
2. Create src/realtime/realtime.service.ts
3. WebSocket gateway as alternative
4. Document integration

Files To Touch

• src/realtime/ (new module)
• src/indexer/event-handlers/
• src/app.module.ts

Acceptance Criteria

• Loan status changes broadcast in real-time
• Payment confirmations broadcast immediately
• WebSocket gateway functional
• npm run build passes

Mandatory Checks Before PR

• npm run build passes
• No any types
• PR references this issue