feat: add locked new-year rollover wizard for season transitions

Promote returning students, assign officers, and clear last season's data after an automatic SQLite backup, with Docker volume path docs fixed for /app/Data.

Co-authored-by: Cursor <cursoragent@cursor.com>

WebApp/Services/IDatabaseBackupService.cs

@@ -0,0 +1,13 @@
+namespace WebApp.Services;
+
+/// <summary>
+/// Creates SQLite database backups.
+/// </summary>
+public interface IDatabaseBackupService
+{
+    /// <summary>
+    /// Creates a pre-rollover backup of the application database using SQLite VACUUM INTO.
+    /// </summary>
+    /// <returns>The absolute path of the backup file.</returns>
+    Task<string> CreatePreRolloverBackupAsync(CancellationToken cancellationToken = default);
+}