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/database/models.go

@@ -80,5 +80,6 @@ type User struct {
 	Username         string
 	Verified         *bool
 	RegistrationDate pgtype.Timestamp
+	Role             int32
 	Deleted          *bool
 }