[BE-061] Yield API: Redis cache invalidation on vault updates

[ONEONUORA]

Description\nEvict platform yield cache immediately when on-chain yield index events are received.\n\n## Files to Edit/Create\n- backend/src/api/yield.rs\n\n## Acceptance Criteria\n- Clear Redis cache keys on receipt of YieldAccrued block events.\n\n## Guidance / Hints\nIssue keys delete command to Redis pool.