[Backend] Create API response caching layer

[SYLVIANNORUKA]

Description: Implement Redis-backed caching for frequently requested read endpoints such as course browsing.

Requirements:

• Handle cache read and write on eligible routes
• Track cache hit and miss rates
• Validate cache key generation per query params
• Manage cache invalidation on content updates
• Provide cache status in response headers

Acceptance Criteria:

• Read and write are handled
• Hit and miss rates are tracked
• Key generation is validated
• Invalidation on updates works
• Cache status is provided in headers

Technical Details:

Files: backend/src/modules/../common/cache/cache.module.ts, backend/src/modules/courses/courses.service.ts
Libraries: ioredis, @nestjs/cache-manager