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/index.ts

@@ -1,3 +1,4 @@
+export { Cacheable, type CacheableOptions } from './decorators/cacheable.decorator';
 export { PrismaService } from './prisma.service';
 export { RedisService } from './redis.service';
 export { CacheService, CachePrefix, CacheTTL } from './cache.service';