feat: add rate limiting to authentication endpoints

- Add rate limiter middleware (10 requests/minute per IP)
- Apply rate limiting to POST /api/auth/register and /api/auth/login
- Prevents brute force attacks and registration spam
- Automatic cleanup of old request records

Closes security issue: No rate limiting on auth endpoints

internal/middleware/rate_limiter.go

@@ -0,0 +1,116 @@
+package middleware
+
+import (
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/labstack/echo/v4"
+)
+
+// RateLimiterConfig defines rate limiting configuration
+type RateLimiterConfig struct {
+	RequestsPerMinute int
+	CleanupInterval   time.Duration
+}
+
+// DefaultRateLimiterConfig returns sensible defaults
+func DefaultRateLimiterConfig() RateLimiterConfig {
+	return RateLimiterConfig{
+		RequestsPerMinute: 10,
+		CleanupInterval:   5 * time.Minute,
+	}
+}
+
+// RateLimiter tracks request counts per IP
+type RateLimiter struct {
+	mu       sync.RWMutex
+	requests map[string][]time.Time
+	config   RateLimiterConfig
+}
+
+// NewRateLimiter creates a new rate limiter
+func NewRateLimiter(config RateLimiterConfig) *RateLimiter {
+	rl := &RateLimiter{
+		requests: make(map[string][]time.Time),
+		config:   config,
+	}
+
+	// Start cleanup goroutine
+	go rl.cleanup()
+
+	return rl
+}
+
+// cleanup removes old request records
+func (rl *RateLimiter) cleanup() {
+	ticker := time.NewTicker(rl.config.CleanupInterval)
+	defer ticker.Stop()
+
+	for range ticker.C {
+		rl.mu.Lock()
+		now := time.Now()
+		for ip, requests := range rl.requests {
+			// Remove requests older than 1 minute
+			var valid []time.Time
+			for _, reqTime := range requests {
+				if now.Sub(reqTime) < time.Minute {
+					valid = append(valid, reqTime)
+				}
+			}
+			if len(valid) == 0 {
+				delete(rl.requests, ip)
+			} else {
+				rl.requests[ip] = valid
+			}
+		}
+		rl.mu.Unlock()
+	}
+}
+
+// Allow checks if a request from the given IP should be allowed
+func (rl *RateLimiter) Allow(ip string) bool {
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	now := time.Now()
+
+	// Clean old requests for this IP
+	var valid []time.Time
+	for _, reqTime := range rl.requests[ip] {
+		if now.Sub(reqTime) < time.Minute {
+			valid = append(valid, reqTime)
+		}
+	}
+	rl.requests[ip] = valid
+
+	// Check if limit exceeded
+	if len(rl.requests[ip]) >= rl.config.RequestsPerMinute {
+		return false
+	}
+
+	// Add this request
+	rl.requests[ip] = append(rl.requests[ip], now)
+	return true
+}
+
+// RateLimiterMiddleware returns echo middleware for rate limiting
+func RateLimiterMiddleware(rl *RateLimiter) echo.MiddlewareFunc {
+	return func(next echo.HandlerFunc) echo.HandlerFunc {
+		return func(c echo.Context) error {
+			// Get client IP
+			ip := c.RealIP()
+			if ip == "" {
+				ip = c.Request().RemoteAddr
+			}
+
+			if !rl.Allow(ip) {
+				return c.JSON(http.StatusTooManyRequests, map[string]string{
+					"error": "too many requests, please try again later",
+				})
+			}
+
+			return next(c)
+		}
+	}
+}