feat: add Redis caching for recipe queries with automatic invalidation

Implements Redis caching layer for recipe endpoints to reduce MongoDB load and improve response times:

- Install ioredis for Redis client with TypeScript support
- Create cache.ts with namespaced keys (homepage: prefix) to avoid conflicts with other Redis applications
- Add caching to recipe query endpoints (all_brief, by tag, in_season) with 1-hour TTL
- Implement automatic cache invalidation on recipe create/edit/delete operations
- Cache recipes before randomization to maximize cache reuse while maintaining random order per request
- Add graceful fallback to MongoDB if Redis is unavailable
- Update .env.example with Redis configuration (REDIS_HOST, REDIS_PORT)

Co-Authored-By: Claude Sonnet 4.5 <noreply@anthropic.com>

.env.example

@@ -1,6 +1,10 @@
 # Database Configuration
 MONGO_URL="mongodb://user:password@host:port/database?authSource=admin"
 
+# Redis Cache Configuration (optional - falls back to direct DB queries if unavailable)
+REDIS_HOST="localhost"  # Redis server hostname
+REDIS_PORT="6379"       # Redis server port
+
 # Authentication Secrets (runtime only - not embedded in build)
 AUTHENTIK_ID="your-authentik-client-id"
 AUTHENTIK_SECRET="your-authentik-client-secret"