prepare for multiple apps

apps/weblibre/lib/data/database/converters/date_time_range.dart

@@ -0,0 +1,46 @@
+/*
+ * Copyright (c) 2024-2026 Fabian Freund.
+ *
+ * This file is part of WebLibre
+ * (see https://weblibre.eu).
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as
+ * published by the Free Software Foundation, either version 3 of the
+ * License, or (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+import 'package:flutter/material.dart';
+import 'package:json_annotation/json_annotation.dart';
+
+class DateTimeRangeConverter
+    implements JsonConverter<DateTimeRange?, Map<String, dynamic>?> {
+  const DateTimeRangeConverter();
+
+  @override
+  DateTimeRange? fromJson(Map<String, dynamic>? json) {
+    if (json == null) return null;
+
+    return DateTimeRange(
+      start: DateTime.parse(json['start'] as String),
+      end: DateTime.parse(json['end'] as String),
+    );
+  }
+
+  @override
+  Map<String, dynamic>? toJson(DateTimeRange? dateTimeRange) {
+    if (dateTimeRange == null) return null;
+
+    return {
+      'start': dateTimeRange.start.toIso8601String(),
+      'end': dateTimeRange.end.toIso8601String(),
+    };
+  }
+}