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/IYearRolloverService.cs

@@ -0,0 +1,36 @@
+using Core.YearTransition;
+
+namespace WebApp.Services;
+
+/// <summary>
+/// Options for applying a year rollover.
+/// </summary>
+public sealed class YearRolloverOptions
+{
+    public required YearTransitionPlan Plan { get; init; }
+    public bool ClearEventOccurrences { get; init; } = true;
+}
+
+/// <summary>
+/// Result of a successful year rollover.
+/// </summary>
+public sealed class YearRolloverResult
+{
+    public required string BackupPath { get; init; }
+    public required int StudentsRemoved { get; init; }
+    public required int StudentsPromoted { get; init; }
+    public required int TeamsDeleted { get; init; }
+    public required int RankingsDeleted { get; init; }
+    public required int MeetingHistoriesDeleted { get; init; }
+    public required int EventOccurrencesDeleted { get; init; }
+    public required string CompetitionYear { get; init; }
+    public required IReadOnlyList<string> OfficerSummary { get; init; }
+}
+
+/// <summary>
+/// Applies a year-transition plan to the database.
+/// </summary>
+public interface IYearRolloverService
+{
+    Task<YearRolloverResult> ApplyAsync(YearRolloverOptions options, CancellationToken cancellationToken = default);
+}