feat(server): split file storage into a separate file service with tickets, receipts and WebDAV backup

- new projects WebDrop.FileApi (file service), WebDrop.FileProtocol (shared tickets and receipts) and WebDrop.Sync (WebDAV)
- file content of any size goes to the file service; chat, avatars and space structure stay on the main backend
- tickets issued by the main backend, receipts by the file service, deletion queue and daily reconciliation
- daily database snapshot and file sync to external storage (OpenList/WebDAV) with a 30 day retention
- reorganize Core, Storage and Api into feature folders (namespaces unchanged)
- web client uploads file frames to the file service and reads them with tickets

server/src/WebDrop.Api/Hosting/BackupWorker.cs

@@ -0,0 +1,65 @@
+using System.IO.Compression;
+using System.Globalization;
+using WebDrop.Storage;
+using WebDrop.Sync;
+
+namespace WebDrop.Api;
+
+/// <summary>
+/// 每天把数据库快照(聊天、头像、账号)压缩后上传到外部存储,只在本地留一个临时文件。
+/// 外部存储里按日期留 RetainDays 份:用户删掉的聊天,最多再在备份里留这么久。
+/// </summary>
+public sealed class BackupWorker(WebDropOptions options, SqliteChannelStore store, ILogger<BackupWorker> logger) : BackgroundService
+{
+    private readonly HttpClient _http = new() { Timeout = TimeSpan.FromMinutes(30) };
+    private const string Prefix = "webdrop-";
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        var backup = options.Backup;
+        if (!backup.Enabled || !backup.WebDav.Configured) return;
+        while (!stoppingToken.IsCancellationRequested)
+        {
+            try { await Task.Delay(DailySchedule.Next(DateTime.Now, backup.TimeOfDay) - DateTime.Now, stoppingToken).ConfigureAwait(false); } catch (OperationCanceledException) { break; }
+            try { await RunOnceAsync(DateTime.Now, stoppingToken).ConfigureAwait(false); }
+            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested) { break; }
+            catch (Exception e) { logger.LogWarning("数据库备份失败:{Type}", e.GetType().Name); }
+        }
+    }
+
+    public async Task<(string Name, int Purged)> RunOnceAsync(DateTime now, CancellationToken ct)
+    {
+        var backup = options.Backup;
+        var dav = new WebDavClient(_http, backup.WebDav);
+        string tmp = Path.Combine(options.DataDir, "v2", "backup-tmp");
+        Directory.CreateDirectory(tmp);
+        string name = $"{Prefix}{now:yyyyMMdd-HHmm}.db.gz";
+        string db = Path.Combine(tmp, $"snapshot-{Guid.NewGuid():N}.db"), gz = db + ".gz";
+        try
+        {
+            store.Snapshot(db);
+            await using (var source = File.OpenRead(db))
+            await using (var target = File.Create(gz))
+            await using (var zip = new GZipStream(target, CompressionLevel.SmallestSize))
+                await source.CopyToAsync(zip, ct).ConfigureAwait(false);
+            await using (var upload = File.OpenRead(gz))
+                await dav.PutAsync($"chat/{name}", upload, ct).ConfigureAwait(false);
+        }
+        finally
+        {
+            foreach (var path in new[] { db, gz }) try { File.Delete(path); } catch (IOException) { }
+        }
+
+        // 保留期:按文件名里的日期清理更早的
+        int purged = 0;
+        var cutoff = now.Date.AddDays(-Math.Max(0, backup.RetainDays));
+        foreach (var existing in await dav.ListAsync("chat", ct).ConfigureAwait(false))
+        {
+            if (!existing.StartsWith(Prefix, StringComparison.Ordinal) || existing.Length < Prefix.Length + 8) continue;
+            if (!DateTime.TryParseExact(existing.AsSpan(Prefix.Length, 8), "yyyyMMdd", CultureInfo.InvariantCulture, DateTimeStyles.None, out var date) || date >= cutoff) continue;
+            if (await dav.DeleteAsync($"chat/{existing}", ct).ConfigureAwait(false)) purged++;
+        }
+        if (purged > 0) logger.LogInformation("清理了 {Count} 份过期的数据库备份", purged);
+        return (name, purged);
+    }
+}