feat: Enhanced rate limiting system

Implements issue #15 - API Rate Limiting

Features:
- Multi-tier rate limiting:
  - Global rate limiting across all clients
  - Per-IP rate limiting
  - Per-user/API key rate limiting
  - Per-endpoint rate limiting with custom limits

- Configurable limits per endpoint:
  - Health check: 600/min (permissive)
  - Search: 60/min (moderate)
  - Create server: 30/min (strict)
  - Delete server: 10/min (very strict)
  - Security tests: 10/min (resource intensive)
  - Auth login: 20/min (brute force prevention)
  - Auth register: 10/min (spam prevention)

- Rate limit response headers:
  - X-RateLimit-Limit
  - X-RateLimit-Remaining
  - X-RateLimit-Reset
  - Retry-After

- Memory management:
  - Automatic cleanup of stale entries
  - Configurable TTL for entries
  - Stats endpoint for monitoring

- Management API:
  - GET /api/v1/rate-limit/stats
  - GET /api/v1/rate-limit/config
  - PUT /api/v1/rate-limit/config

Closes #15

Author: bad-antics