core: implement sponsor yield calculation service

[EmeditWeb]

Problem

Sponsors have no way to see their earned yield.
The API does not calculate yield based on share
price changes.

What To Build

1. Create src/modules/yield/yield.service.ts
2. GET /sponsors/yield endpoint
3. Daily yield snapshots via BullMQ

Files To Touch

• src/modules/yield/ (new)
• src/modules/sponsors/sponsors.controller.ts
• supabase/migrations/[ts]_yield_snapshots.sql

Acceptance Criteria

• Yield calculated correctly from share price
• APY calculation uses actual time held
• Daily snapshots stored
• History available for charts
• npm run build passes

Mandatory Checks Before PR

• npm run build passes
• No any types
• Unit tests for yield calculation
• PR references this issue