feat(backend): initial data export support

obviously it's missing things that haven't been added yet

Foxnouns.Backend/Services/DataCleanupService.cs

@@ -0,0 +1,47 @@
+using Foxnouns.Backend.Database;
+using Foxnouns.Backend.Database.Models;
+using Microsoft.EntityFrameworkCore;
+using NodaTime;
+
+namespace Foxnouns.Backend.Services;
+
+public class DataCleanupService(
+    DatabaseContext db,
+    IClock clock,
+    ILogger logger,
+    ObjectStorageService objectStorageService
+)
+{
+    private readonly ILogger _logger = logger.ForContext<DataCleanupService>();
+
+    public async Task InvokeAsync(CancellationToken ct = default)
+    {
+        _logger.Information("Cleaning up expired data exports");
+        await CleanExportsAsync(ct);
+    }
+
+    private async Task CleanExportsAsync(CancellationToken ct = default)
+    {
+        var minExpiredId = Snowflake.FromInstant(clock.GetCurrentInstant() - DataExport.Expiration);
+        var exports = await db.DataExports.Where(d => d.Id < minExpiredId).ToListAsync(ct);
+        if (exports.Count == 0)
+            return;
+
+        _logger.Debug("There are {Count} expired exports", exports.Count);
+
+        foreach (var export in exports)
+        {
+            _logger.Debug("Deleting export {ExportId}", export.Id);
+            await objectStorageService.RemoveObjectAsync(
+                ExportPath(export.UserId, export.Filename),
+                ct
+            );
+            db.Remove(export);
+        }
+
+        await db.SaveChangesAsync(ct);
+    }
+
+    private static string ExportPath(Snowflake userId, string b64) =>
+        $"data-exports/{userId}/{b64}.zip";
+}