feat: implement Redis cache invalidation strategy

[elijah4196]

feat: implement Redis cache invalidation strategy

Summary

Implements a coherent Redis cache invalidation strategy across the ProxyPay API, addressing inconsistent TTLs and missing invalidation hooks for user-specific data. This PR introduces event-driven invalidation for user caches, corrects reference data TTLs, and adds an admin endpoint for emergency cache flushing with full structured audit logging.

Changes

1. Reduce fee config TTL (src/services/feeService.ts)

• Changed CACHE_TTL from 3600 (1 hour) to 600 (10 minutes) so fee changes propagate across instances within an acceptable window.

2. Cache Invalidation Logger (src/utils/cacheInvalidationLogger.ts)

• New utility module exporting CacheInvalidationEvent interface, CacheInvalidationTrigger union type, and logCacheInvalidation() function.
• All cache invalidation events emit a structured pino log entry: { event: "cache_invalidated", key?, pattern?, trigger, adminId?, timestamp }.

3. KYC status cache invalidation (src/services/kyc.ts)

• updateUserKYCLevel() now invalidates cache:kyc:{userId} via layeredCache.del() immediately after the DB write succeeds.
• Cache errors are non-fatal (logged at warn, not re-thrown).
• Emits a cache_invalidated log with trigger: "kyc_level_update".

4. API key cache tag (src/services/cachedQueryManager.ts)

• Added CacheTags.apiKeys(userId) static method returning user:{userId}:apikeys.

5. API key cache invalidation contract (src/services/apiKeyService.ts)

• New service documenting the cache:apikeys:{userId} key pattern (TTL 600s) and exporting invalidateApiKeyCache(userId) for use at mutation sites when API key CRUD is implemented.

6. Fee service invalidation logging (src/services/feeService.ts)

• invalidateCache(id) and invalidateAllCaches() now call logCacheInvalidation() with trigger: "fee_config_change" after flushing from LayeredCache.

7. Admin cache flush endpoint (src/routes/admin.ts)

• Added POST /api/admin/cache/flush?key={pattern} route protected by requireAdmin middleware.
• Calls layeredCache.delPattern(pattern) and emits a cache_invalidated log with trigger: "admin_flush" and the admin's user ID.
• Returns { flushed: true, pattern } on success; 400 if key param is missing/empty; 403 if caller is not admin.

Cache Key Registry

Cache	Key Pattern	TTL	Invalidation
KYC status	cache:kyc:{userId}	300s	Event-driven on updateUserKYCLevel
API key list	cache:apikeys:{userId}	600s	Event-driven on API key mutation
Fee config (id)	fee_config:{id}	600s	Event-driven on FeeService write
Fee config (active)	fee_config:active	600s	Event-driven on FeeService write
Country list	cache:country:list	900s	TTL expiry only
Asset metadata	cache:asset:metadata:{assetCode}	900s	TTL expiry only

Requirements Addressed

• Requirement 1 — KYC status cache invalidated on updateUserKYCLevel ✅
• Requirement 2 — API key cache key format defined; invalidation contract documented ✅
• Requirement 3 — Fee config TTL reduced to 10 min; invalidation logging added ✅
• Requirement 4 — Country list cache key/TTL defined (300s) ✅
• Requirement 5 — Asset metadata cache key/TTL defined (900s) ✅
• Requirement 6 — POST /admin/cache/flush endpoint with auth, logging, validation ✅
• Requirement 7 — CacheInvalidationLogger with structured pino events for all triggers ✅

Migration Notes

All changes are fully backward-compatible. No database migrations required. The only behavioral change visible to existing code is the reduced fee config TTL (3600s → 600s), which means cached fee configs will refresh more frequently — this is intentional and safe.

Testing

All changes are additive and non-breaking. Existing test suites continue to pass. The implementation uses:

• The existing layeredCache L1+L2 infrastructure (no new Redis connections)
• The existing pino logger (no new logging dependencies)
• The existing requireAdmin middleware for the flush endpoint

closes #106

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