feat: add user role support to database and queries;

fix: add max length validation for refresh token in RefreshRequest;
refactor: use named constants for cache durations in AuthService;
refactor: select all user columns in GetValidUserByLoginCredentials query;

backend/internal/models/auth.go

@@ -49,9 +49,8 @@ type LoginResponse struct {
 	Tokens
 }
 
-// TODO: length check
 type RefreshRequest struct {
-	RefreshToken string `json:"refresh_token" binding:"required"`
+	RefreshToken string `json:"refresh_token" binding:"required,max=2000"`
 }
 
 type RefreshResponse struct {