🗃️ Add more cache ability to local database

lib/database/attachment.dart

@@ -0,0 +1,45 @@
+import 'dart:convert';
+
+import 'package:drift/drift.dart';
+import 'package:surface/types/attachment.dart';
+
+class SnAttachmentConverter extends TypeConverter<SnAttachment, String>
+    with JsonTypeConverter2<SnAttachment, String, Map<String, Object?>> {
+  const SnAttachmentConverter();
+
+  @override
+  SnAttachment fromSql(String fromDb) {
+    return fromJson(jsonDecode(fromDb) as Map<String, dynamic>);
+  }
+
+  @override
+  String toSql(SnAttachment value) {
+    return jsonEncode(toJson(value));
+  }
+
+  @override
+  SnAttachment fromJson(Map<String, Object?> json) {
+    return SnAttachment.fromJson(json);
+  }
+
+  @override
+  Map<String, Object?> toJson(SnAttachment value) {
+    return value.toJson();
+  }
+}
+
+@TableIndex(name: 'idx_attachment_rid', columns: {#rid})
+@TableIndex(name: 'idx_attachment_account', columns: {#accountId})
+class SnLocalAttachment extends Table {
+  IntColumn get id => integer().autoIncrement()();
+
+  TextColumn get rid => text().unique()();
+
+  TextColumn get uuid => text().unique()();
+
+  TextColumn get content => text().map(const SnAttachmentConverter())();
+
+  IntColumn get accountId => integer()();
+
+  DateTimeColumn get createdAt => dateTime().withDefault(currentDateAndTime)();
+}