Add Redis Caching to Next.js (Cursor Prompt)

Add Redis caching to this Next.js app using Upstash Redis.

FILES TO CREATE:
- src/lib/redis.ts — Redis client initialization
- src/lib/cache.ts — Cache utility with get, set, delete, and invalidation patterns

FILES TO MODIFY:
- Existing query functions — wrap with caching layer

IMPLEMENTATION:
1. Install @upstash/redis.
2. In redis.ts, create Redis client with UPSTASH_REDIS_REST_URL and UPSTASH_REDIS_REST_TOKEN.
3. In cache.ts, implement:
   - cacheGet<T>(key: string): Promise<T | null> — Get cached value with type safety
   - cacheSet(key: string, value: any, ttlSeconds: number) — Set with TTL
   - cacheDelete(key: string) — Delete single key
   - cacheDeletePattern(pattern: string) — Delete keys matching pattern (e.g., 'posts:*')
   - withCache<T>(key: string, ttl: number, fetcher: () => Promise<T>): Promise<T> — Cache-aside wrapper
4. Use the withCache wrapper in data fetching functions:
   - Cache expensive database queries (listing pages, aggregations)
   - Use TTL based on data freshness needs (5 min for listings, 1 hour for settings)
5. Invalidate cache after mutations:
   - After creating/updating a post, delete 'posts:*' cache keys
   - After updating settings, delete 'settings:*' cache keys

CACHE KEY CONVENTION:
- Format: {resource}:{identifier}:{variant} (e.g., posts:list:page-1, posts:detail:abc-123, user:profile:user-id)

DO NOT:
- Cache user-specific authenticated data without including user ID in the key
- Set TTL longer than 24 hours for dynamic content
- Cache error responses

ENVIRONMENT VARIABLES:
- UPSTASH_REDIS_REST_URL
- UPSTASH_REDIS_REST_TOKEN