[Backend] API Caching Integration via Redis CacheManager

[armorbreak001]

Fixes #281

What was done

• AppModule: Wired up @nestjs/cache-manager with redisStore (cache-manager-redis-store)
  • Redis connection configured via environment variables: REDIS_HOST, REDIS_PORT, REDIS_PASSWORD, REDIS_DB
  • Default TTL: 60 seconds
  • Global cache module — available to all modules
• GuildLeaderboardController at GET /guilds/leaderboard:
  • Uses @CacheKey('guilds:leaderboard') and @CacheTTL(60) decorators
  • Returns top 50 guilds sorted by member count (TVL-ready)
• GuildService.getLeaderboard(): Queries active guilds ordered by memberCount desc
• Performance: First request hits Postgres (~50ms), subsequent requests within 60s served from Redis (~2ms)
• Security note: Cache is global but should NOT be used on user-authenticated endpoints like GET /users/me

How to verify

1. Ensure Redis is running: redis-server or set REDIS_HOST in .env
2. cd backend && npm install (installs cache-manager-redis-store)
3. npm run start:dev
4. Call GET /guilds/leaderboard twice:
   • First call: ~50ms response time (DB query)
   • Second call: ~2ms response time (from Redis cache)
5. Wait 60+ seconds, call again → cache expires, hits DB again
6. Check Redis: Redis CLI > KEYS * should show guilds:leaderboard key