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.Core/Files/IFileGateway.cs

@@ -0,0 +1,28 @@
+using WebDrop.FileProtocol;
+
+namespace WebDrop.Core;
+
+/// <summary>给客户端的文件服务入口:地址 + 一张短期票据</summary>
+public sealed record FileGrant(string Url, string Ticket, long ExpiresAt);
+
+/// <summary>
+/// 主后端和文件服务之间的接口。关闭时(默认)所有帧仍然存在主后端自己的存储里,行为和以前完全一样;
+/// 打开后,文件的内容(文件域的对象,不分大小)由客户端直接传给文件服务,主后端只登记元数据和摘要;聊天、头像、目录仍在主后端。
+/// </summary>
+public interface IFileGateway
+{
+    bool Enabled { get; }
+    /// <summary>客户端拿到的文件服务根地址(经反向隧道对外的那个)</summary>
+    string PublicUrl { get; }
+    FileGrant Issue(FileOp op, Id16 channel, Id16 item, uint from, long nowMs);
+    bool TryVerifyReceipt(string? token, long nowMs, out FileReceipt receipt);
+}
+
+public sealed class NoFileGateway : IFileGateway
+{
+    public static readonly NoFileGateway Instance = new();
+    public bool Enabled => false;
+    public string PublicUrl => "";
+    public FileGrant Issue(FileOp op, Id16 channel, Id16 item, uint from, long nowMs) => throw new WebDropException(ErrorCode.NotFound);
+    public bool TryVerifyReceipt(string? token, long nowMs, out FileReceipt receipt) { receipt = null!; return false; }
+}