fix(backend): rate-limit login and refresh endpoints

/auth/login and /auth/refresh had no throttling, allowing unbounded
password brute-force attempts. Add a process-local fixed-window limiter
(10 requests/minute per client IP) in front of both.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/internal/handler/ratelimit.go

@@ -0,0 +1,77 @@
+package handler
+
+import (
+	"net/http"
+	"sync"
+	"time"
+
+	"github.com/gin-gonic/gin"
+)
+
+// rateLimiter is a process-local, fixed-window per-key request limiter used to
+// throttle unauthenticated endpoints (login, refresh) against brute force. It
+// is best-effort: counts live in memory and reset on restart.
+type rateLimiter struct {
+	mu     sync.Mutex
+	counts map[string]*rateWindow
+	limit  int
+	window time.Duration
+}
+
+type rateWindow struct {
+	count int
+	reset time.Time
+}
+
+// newRateLimiter allows up to limit requests per key within each window.
+func newRateLimiter(limit int, window time.Duration) *rateLimiter {
+	return &rateLimiter{
+		counts: make(map[string]*rateWindow),
+		limit:  limit,
+		window: window,
+	}
+}
+
+// allow records a request for key and reports whether it is within the limit.
+func (rl *rateLimiter) allow(key string) bool {
+	now := time.Now()
+
+	rl.mu.Lock()
+	defer rl.mu.Unlock()
+
+	// Opportunistically prune expired entries so the map cannot grow without
+	// bound under a flood of distinct client IPs.
+	if len(rl.counts) > 10000 {
+		for k, w := range rl.counts {
+			if now.After(w.reset) {
+				delete(rl.counts, k)
+			}
+		}
+	}
+
+	w, ok := rl.counts[key]
+	if !ok || now.After(w.reset) {
+		rl.counts[key] = &rateWindow{count: 1, reset: now.Add(rl.window)}
+		return true
+	}
+	if w.count >= rl.limit {
+		return false
+	}
+	w.count++
+	return true
+}
+
+// Middleware throttles requests by client IP, returning 429 when over the limit.
+func (rl *rateLimiter) Middleware() gin.HandlerFunc {
+	return func(c *gin.Context) {
+		if !rl.allow(c.ClientIP()) {
+			c.JSON(http.StatusTooManyRequests, errorBody{
+				Code:    "rate_limited",
+				Message: "too many requests, please try again later",
+			})
+			c.Abort()
+			return
+		}
+		c.Next()
+	}
+}