Issue #60 — Cache invalidation service

[GoSTEAN]

Title: [Repository] Implement centralised cache invalidation service

Tags: backend, repository, feature

Description:
Centralised service to invalidate Redis cache keys when underlying data changes, preventing stale reads.

Acceptance Criteria:

• cache.service.ts exposes: get, set, del, delPattern, getOrSet.
• delPattern('market:*') purges all market cache entries using Redis SCAN.
• Cache keys namespaced: market:{id}, leaderboard:global:*, user:{id}:balance.
• After any write in market.service.ts, wallet.service.ts, leaderboard.service.ts, the relevant cache keys are invalidated.
• Unit test: set → get → invalidate → get returns null.

File: src/services/cache.service.ts

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