fix: use constant time in user lookups

internal/controller/user_controller.go

@@ -89,21 +89,30 @@ func (controller *UserController) loginHandler(c *gin.Context) {
 	search, err := controller.auth.SearchUser(req.Username)
 
 	if err != nil {
+		controller.constantTime(func() constantTimeRes {
 			if errors.Is(err, service.ErrUserNotFound) {
 				controller.log.App.Warn().Str("username", req.Username).Msg("User not found during login attempt")
 				controller.auth.RecordLoginAttempt(req.Username, false)
 				controller.log.AuditLoginFailure(req.Username, "unknown", c.ClientIP(), "user not found")
-			c.JSON(401, gin.H{
+				return constantTimeRes{
+					Code: 401,
+					Res: gin.H{
 						"status":  401,
 						"message": "Unauthorized",
-			})
+					},
-			return
+				}
 			}
 			controller.log.App.Error().Err(err).Str("username", req.Username).Msg("Error searching for user during login attempt")
-		c.JSON(500, gin.H{
+			return constantTimeRes{
+				Code: 500,
+				Res: gin.H{
 					"status":  500,
 					"message": "Internal Server Error",
-		})
+				},
+			}
+		}, func(res constantTimeRes) {
+			c.JSON(res.Code, res.Res)
+		}, time.Millisecond*45)
 		return
 	}
 
@@ -466,3 +475,18 @@ func (controller *UserController) tailscaleHandler(c *gin.Context) {
 		"message": "Login successful",
 	})
 }
+
+type constantTimeRes struct {
+	Code int
+	Res  any
+}
+
+func (controller *UserController) constantTime(f func() constantTimeRes, rf func(res constantTimeRes), targetTime time.Duration) {
+	tStart := time.Now()
+	res := f()
+	tEnd := time.Now()
+	if tEnd.Sub(tStart) < targetTime {
+		time.Sleep(targetTime - tEnd.Sub(tStart))
+	}
+	rf(res)
+}