feat: add Redis caching for bounty listings (#128)

[TeapoyY]

Description

Fixes issue #128: Redis caching for the bounty listings.

Added Redis caching to the GET /api/bounties endpoint to improve performance for frequently-accessed bounty listings.

Changes

New dependency

• Added ioredis ^5.6.1 to @devasign/api package

New service: packages/api/src/services/redis.ts

• getRedisClient() - Lazy singleton Redis client using REDIS_URL env var
• getCached<T>(key) - Get and parse JSON from Redis, returns null if Redis unavailable
• setCache(key, value, ttlSeconds) - Set a JSON value with TTL
• deleteCachePattern(pattern) - Scan and delete all keys matching a pattern (non-blocking)

Cache implementation in GET /api/bounties

• Cache key is built deterministically from query params (tech_stack, amount_min, amount_max, difficulty, status, limit)
• First page (no cursor) results are cached for 30 seconds (BOUNTY_LIST_CACHE_TTL)
• Subsequent pages (with cursor) are not cached to keep pagination consistent
• Cache is automatically invalidated when a bounty is updated or completed

Cache invalidation

• PATCH /api/bounties/:id - Invalidates all bounty:list:* keys
• POST /api/bounties/:id/complete - Invalidates all bounty:list:* keys

Testing

1. Start Redis via docker-compose up redis
2. Ensure REDIS_URL=redis://localhost:6379 in .env
3. Request GET /api/bounties - first request hits DB, second request should be served from Redis cache
4. Update a bounty - next listing request should hit DB again (cache invalidated)

Closes #128