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/IChapterSettingsWriter.cs

@@ -0,0 +1,19 @@
+using WebApp.Models;
+
+namespace WebApp.Services;
+
+/// <summary>
+/// Persists chapter settings to <c>Data/appsettings.json</c>.
+/// </summary>
+public interface IChapterSettingsWriter
+{
+    /// <summary>
+    /// Writes the given chapter settings, preserving other top-level sections in the file.
+    /// </summary>
+    Task WriteAsync(ChapterSettings settings, CancellationToken cancellationToken = default);
+
+    /// <summary>
+    /// Updates only the competition year while preserving other chapter settings from configuration.
+    /// </summary>
+    Task UpdateCompetitionYearAsync(string competitionYear, CancellationToken cancellationToken = default);
+}