refactor: move backend files to backend/ directory

Reorganize project structure:
- backend/cmd/openteam/ — entry point
- backend/internal/ — core packages
- backend/middleware/ — HTTP middleware
- backend/router/ — route setup
- backend/wire/ — dependency injection
- backend/pkg/ — shared utilities
- backend/go.mod, go.sum — Go module files

Updated Makefile to work from backend/ directory.
Removed old lowercase makefile.

backend/middleware/ratelimit.go

@@ -0,0 +1,53 @@
+package middleware
+
+import (
+	"net/http"
+	"sync"
+
+	"github.com/gin-gonic/gin"
+	"golang.org/x/time/rate"
+)
+
+type IPRateLimiter struct {
+	ips map[string]*rate.Limiter
+	mu  *sync.RWMutex
+	r   rate.Limit
+	b   int
+}
+
+func NewIPRateLimiter(r rate.Limit, b int) *IPRateLimiter {
+	return &IPRateLimiter{
+		ips: make(map[string]*rate.Limiter),
+		mu:  &sync.RWMutex{},
+		r:   r,
+		b:   b,
+	}
+}
+
+func (i *IPRateLimiter) GetLimiter(ip string) *rate.Limiter {
+	i.mu.Lock()
+	defer i.mu.Unlock()
+
+	limiter, exists := i.ips[ip]
+	if !exists {
+		limiter = rate.NewLimiter(i.r, i.b)
+		i.ips[ip] = limiter
+	}
+
+	return limiter
+}
+
+func RateLimit(limiter *IPRateLimiter) gin.HandlerFunc {
+	return func(c *gin.Context) {
+		ip := c.ClientIP()
+		if !limiter.GetLimiter(ip).Allow() {
+			c.JSON(http.StatusTooManyRequests, gin.H{
+				"code":    429,
+				"message": "too many requests",
+			})
+			c.Abort()
+			return
+		}
+		c.Next()
+	}
+}