feat: implement database backup and restore functionality with JSON support

Co-authored-by: Copilot <copilot@github.com>

Services/Storage/DatabaseBackupModels.cs

@@ -0,0 +1,38 @@
+using System.Text.Json;
+
+namespace WorkTracker.Services.Storage;
+
+public sealed class DatabaseBackupFile
+{
+    public required byte[] Content { get; init; }
+
+    public required string ContentType { get; init; }
+
+    public required string FileName { get; init; }
+}
+
+public sealed class DatabaseBackupPayload
+{
+    public int BackupFormatVersion { get; init; }
+
+    public int DatabaseSchemaVersion { get; init; }
+
+    public DateTimeOffset ExportedAtUtc { get; init; }
+
+    public Dictionary<string, List<DatabaseBackupDocumentPayload>> Collections { get; init; } = [];
+}
+
+public sealed class DatabaseBackupDocumentPayload
+{
+    public string Id { get; init; } = string.Empty;
+
+    public JsonElement Data { get; init; }
+}
+
+public sealed class DatabaseBackupException : Exception
+{
+    public DatabaseBackupException(string message)
+        : base(message)
+    {
+    }
+}