Implemented Redis Caching

[Mkalbani]

closes #118

Redis Caching Integration

Integrate Redis caching for response caching and future rate-limiting in drips.

Changes:

• Install @nestjs/cache-manager, cache-manager, ioredis, cache-manager-redis-store
• Configure CacheModule.registerAsync with Redis store using environment variables (REDIS_HOST, REDIS_PORT, REDIS_PASSWORD, REDIS_DB)
• Add sample cached endpoint GET /cached-data with @UseInterceptors(CacheInterceptor) decorator
• 5-minute default TTL configured for cache entries

Testing:

• Redis connects on app startup via AppConfigService
• Cache hits observable: /cached-data returns same timestamp on repeated calls within TTL window