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/DatabaseBackupService.cs

@@ -0,0 +1,48 @@
+using Data;
+using Microsoft.EntityFrameworkCore;
+
+namespace WebApp.Services;
+
+/// <summary>
+/// Creates SQLite database backups via VACUUM INTO.
+/// </summary>
+public class DatabaseBackupService : IDatabaseBackupService
+{
+    private readonly AppDbContext _context;
+    private readonly IWebHostEnvironment _environment;
+    private readonly ILogger<DatabaseBackupService> _logger;
+
+    public DatabaseBackupService(
+        AppDbContext context,
+        IWebHostEnvironment environment,
+        ILogger<DatabaseBackupService> logger)
+    {
+        _context = context;
+        _environment = environment;
+        _logger = logger;
+    }
+
+    public async Task<string> CreatePreRolloverBackupAsync(CancellationToken cancellationToken = default)
+    {
+        var backupsDir = Path.Combine(_environment.ContentRootPath, "Data", "backups");
+        Directory.CreateDirectory(backupsDir);
+
+        var fileName = $"pre-rollover-{DateTime.Now:yyyyMMdd-HHmmss}.db";
+        var backupPath = Path.Combine(backupsDir, fileName);
+
+        // Path is server-generated (never user input); escape single quotes for SQLite string literal.
+        var escapedPath = backupPath.Replace("'", "''", StringComparison.Ordinal);
+#pragma warning disable EF1002 // Path is fully server-controlled; VACUUM INTO cannot use parameters.
+        await _context.Database.ExecuteSqlRawAsync($"VACUUM INTO '{escapedPath}'", cancellationToken);
+#pragma warning restore EF1002
+
+        if (!File.Exists(backupPath))
+        {
+            throw new InvalidOperationException(
+                $"Database backup was requested but the file was not created at '{backupPath}'.");
+        }
+
+        _logger.LogInformation("Created pre-rollover database backup at {BackupPath}", backupPath);
+        return backupPath;
+    }
+}