feat: add login attempt rate limiting

auth.go

@@ -7,6 +7,7 @@ import (
 	"fmt"
 	"net/http"
 	"strings"
+	"sync"
 	"time"
 
 	"github.com/gin-gonic/gin"
@@ -15,6 +16,30 @@ import (
 
 const sessionCookie = "am_session"
 
+var loginLimiter = struct {
+	sync.Mutex
+	attempts map[string][]time.Time
+}{attempts: map[string][]time.Time{}}
+
+func loginAllowed(key string) bool {
+	now := time.Now()
+	loginLimiter.Lock()
+	defer loginLimiter.Unlock()
+	items := loginLimiter.attempts[key]
+	cutoff := now.Add(-15 * time.Minute)
+	kept := items[:0]
+	for _, t := range items {
+		if t.After(cutoff) {
+			kept = append(kept, t)
+		}
+	}
+	if len(kept) >= 10 {
+		return false
+	}
+	loginLimiter.attempts[key] = append(kept, now)
+	return true
+}
+
 func hashToken(token string) string {
 	sum := sha256.Sum256([]byte(token))
 	return hex.EncodeToString(sum[:])
@@ -104,6 +129,11 @@ func (a *App) login(c *gin.Context) {
 		c.JSON(http.StatusBadRequest, gin.H{"error": "username and password required"})
 		return
 	}
+	key := c.ClientIP() + ":" + strings.ToLower(strings.TrimSpace(in.Username))
+	if !loginAllowed(key) {
+		c.JSON(http.StatusTooManyRequests, gin.H{"error": "too many login attempts; try again later"})
+		return
+	}
 	var user User
 	if a.db.Where("username = ?", strings.TrimSpace(in.Username)).First(&user).Error != nil || user.Status != UserActive || checkPassword(user.PasswordHash, in.Password) != nil {
 		c.JSON(http.StatusUnauthorized, gin.H{"error": "invalid username or password"})
@@ -120,6 +150,7 @@ func (a *App) login(c *gin.Context) {
 		return
 	}
 	http.SetCookie(c.Writer, &http.Cookie{Name: sessionCookie, Value: token, Path: "/", HttpOnly: true, SameSite: http.SameSiteLaxMode, Secure: c.Request.TLS != nil, Expires: session.ExpiresAt})
+	user.PasswordHash = ""
 	c.JSON(http.StatusOK, gin.H{"user": user})
 }