feat(api): add @Cacheable decorator and plan list caching

- Create @Cacheable method decorator for declarative cache-aside pattern
  with configurable prefix, TTL, resource label, and key extraction
- Add PLAN_LIST (1h TTL) and REFERENCE_DATA (24h TTL) cache constants
- Add CachePrefix.PLAN_LIST and CachePrefix.REFERENCE entries
- Cache subscription plan queries in GetPlanHandler (single + list)
- Export Cacheable decorator from shared module barrel
- Add comprehensive tests for decorator and handler caching

The caching infrastructure (CacheService, Redis, Prometheus metrics,
event-driven invalidation) was already production-ready with 10+ hot
paths cached. This commit adds the missing declarative decorator and
plan list caching.

Resolves: TEC-1567

Co-Authored-By: Paperclip <noreply@paperclip.ing>

apps/api/src/modules/shared/infrastructure/cache.service.ts

@@ -24,6 +24,10 @@ export const CacheTTL = {
   USER_PROFILE: 600, // 10 min
   /** User quota — very short TTL, invalidated on usage metering and plan changes */
   USER_QUOTA: 60, // 1 min
+  /** Subscription plan list — long TTL, rarely changes */
+  PLAN_LIST: 3600, // 1 hour
+  /** Reference data (districts, wards) — very long TTL, static data */
+  REFERENCE_DATA: 86400, // 24 hours
 } as const;
 
 export enum CachePrefix {
@@ -37,6 +41,8 @@ export enum CachePrefix {
   USER_PROFILE = 'cache:user:profile',
   USER_QUOTA = 'cache:user:quota',
   VALUATION = 'cache:valuation',
+  PLAN_LIST = 'cache:plan:list',
+  REFERENCE = 'cache:reference',
 }
 
 @Injectable()