Add basic rate limiting middleware #5

The API has no rate limiting. Add per-IP rate limiting using Fiber's built-in limiter middleware.

What to change:

• Add github.com/gofiber/fiber/v2/middleware/limiter to server/cmd/server/main.go
• Configure: 60 requests per minute per IP, return 429 when exceeded
• Add RATE_LIMIT env var to make it configurable (0 = disabled)
• Add to config.go, .env.example, and README config table

Difficulty: Easy — Fiber has built-in middleware, ~20 lines