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.FileApi/SyncWorker.cs

@@ -0,0 +1,70 @@
+using WebDrop.FileProtocol;
+using WebDrop.Sync;
+
+namespace WebDrop.FileApi;
+
+public sealed record SyncSummary(int Uploaded, long UploadedBytes, int Failed, int Purged);
+
+/// <summary>
+/// 每天定时把新增的文件帧同步到外部存储(WebDAV,如 OpenList);删除了的内容在外部存储里再留 RetainDays 天才清理。
+/// 帧的文件名里就是 频道/条目/序号,一帧一个远端文件,重复上传同样内容是幂等的,中途失败下次接着传。
+/// </summary>
+public sealed class SyncWorker(FileApiOptions options, FrameStore store, ILogger<SyncWorker> logger) : BackgroundService
+{
+    private readonly HttpClient _http = new() { Timeout = TimeSpan.FromMinutes(30) };
+
+    protected override async Task ExecuteAsync(CancellationToken stoppingToken)
+    {
+        var backup = options.Backup;
+        if (!backup.Enabled || !backup.WebDav.Configured) return;
+        while (!stoppingToken.IsCancellationRequested)
+        {
+            var wait = DailySchedule.Next(DateTime.Now, backup.TimeOfDay) - DateTime.Now;
+            try { await Task.Delay(wait, stoppingToken).ConfigureAwait(false); } catch (OperationCanceledException) { break; }
+            try { await RunOnceAsync(stoppingToken).ConfigureAwait(false); }
+            catch (OperationCanceledException) when (stoppingToken.IsCancellationRequested) { break; }
+            catch (Exception e) { logger.LogWarning("文件同步失败:{Type}", e.GetType().Name); }
+        }
+    }
+
+    public async Task<SyncSummary> RunOnceAsync(CancellationToken ct)
+    {
+        var backup = options.Backup;
+        var dav = new WebDavClient(_http, backup.WebDav);
+        var throttle = new Throttle(backup.MaxBytesPerSecond);
+        int uploaded = 0, failed = 0, purged = 0; long bytes = 0;
+
+        foreach (var (key, path) in store.Unsynced())
+        {
+            ct.ThrowIfCancellationRequested();
+            var started = DateTime.UtcNow;
+            try
+            {
+                await using var file = File.OpenRead(path);
+                long length = file.Length; // 上传结束后流已经被释放
+                await dav.PutAsync($"files/{key}", file, ct).ConfigureAwait(false);
+                store.MarkSynced(key);
+                uploaded++; bytes += length;
+                await throttle.AfterAsync(length, DateTime.UtcNow - started, ct).ConfigureAwait(false);
+            }
+            catch (Exception e) when (e is HttpRequestException or IOException)
+            {
+                // 外部存储连不上或这一帧读不了(正好被删):记一笔,连续失败几次就等下一次再来
+                if (++failed >= 5) break;
+            }
+        }
+
+        // 保留期已过的删除记录:从外部存储清掉
+        var expired = store.ExpiredTombstones(TimeSpan.FromDays(Math.Max(0, backup.RetainDays)));
+        var cleaned = new List<string>();
+        foreach (var prefix in expired)
+        {
+            try { await dav.DeleteAsync($"files/{prefix}", ct).ConfigureAwait(false); cleaned.Add(prefix); purged++; }
+            catch (HttpRequestException) { break; }
+        }
+        store.RemoveTombstones(cleaned);
+
+        if (uploaded + purged + failed > 0) logger.LogInformation("文件同步:上传 {Uploaded} 帧({Bytes} 字节),失败 {Failed},清理 {Purged}", uploaded, bytes, failed, purged);
+        return new SyncSummary(uploaded, bytes, failed, purged);
+    }
+}