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.Storage/Blobs/DiskBlobStore.cs

@@ -0,0 +1,90 @@
+using WebDrop.Core;
+
+namespace WebDrop.Storage;
+
+/// <summary>
+/// 磁盘 blob 存储。键形如 "channelHex/itemHex/index.digest",直接映射为目录与文件。
+/// 键只能由服务生成(十六进制和数字),这里仍做路径逃逸检查作为纵深防御。
+/// </summary>
+public sealed class DiskBlobStore : IBlobStore
+{
+    private readonly string _root;
+
+    public DiskBlobStore(string root)
+    {
+        _root = Path.GetFullPath(root);
+        Directory.CreateDirectory(_root);
+    }
+
+    public async ValueTask PutAsync(string key, ReadOnlyMemory<byte> data, CancellationToken ct = default)
+    {
+        string path = Resolve(key);
+        // 键里含内容摘要:文件已存在就意味着内容相同,无需重写(也避免并发覆盖在 Windows 上冲突)
+        if (File.Exists(path)) return;
+        Directory.CreateDirectory(Path.GetDirectoryName(path)!);
+
+        // 先写临时文件再重命名,读者永远看不到半个文件
+        string tmp = path + "." + Guid.NewGuid().ToString("N") + ".tmp";
+        await using (var fs = new FileStream(tmp, FileMode.CreateNew, FileAccess.Write, FileShare.None, 4096, FileOptions.Asynchronous))
+        {
+            await fs.WriteAsync(data, ct).ConfigureAwait(false);
+        }
+
+        try
+        {
+            File.Move(tmp, path, overwrite: false);
+        }
+        catch (Exception e) when (e is IOException or UnauthorizedAccessException && File.Exists(path))
+        {
+            File.Delete(tmp); // 并发写入同一内容,对方先到了
+        }
+    }
+
+    public async ValueTask<byte[]?> ReadAsync(string key, CancellationToken ct = default)
+    {
+        string path = Resolve(key);
+        try
+        {
+            return await File.ReadAllBytesAsync(path, ct).ConfigureAwait(false);
+        }
+        catch (FileNotFoundException)
+        {
+            return null;
+        }
+        catch (DirectoryNotFoundException)
+        {
+            return null;
+        }
+    }
+
+    public void Delete(string key)
+    {
+        try { File.Delete(Resolve(key)); }
+        catch (DirectoryNotFoundException) { }
+    }
+
+    public void DeletePrefix(string prefix)
+    {
+        string dir = Resolve(prefix);
+        try { if (Directory.Exists(dir)) Directory.Delete(dir, recursive: true); }
+        catch (DirectoryNotFoundException) { }
+    }
+
+    public IEnumerable<(string Key, DateTimeOffset LastWrite)> Enumerate()
+    {
+        foreach (string file in Directory.EnumerateFiles(_root, "*", SearchOption.AllDirectories))
+        {
+            if (file.EndsWith(".tmp", StringComparison.Ordinal)) continue;
+            string key = Path.GetRelativePath(_root, file).Replace('\\', '/');
+            yield return (key, new DateTimeOffset(File.GetLastWriteTimeUtc(file), TimeSpan.Zero));
+        }
+    }
+
+    private string Resolve(string key)
+    {
+        string full = Path.GetFullPath(Path.Combine(_root, key));
+        if (!full.StartsWith(_root + Path.DirectorySeparatorChar, StringComparison.Ordinal))
+            throw new ArgumentException("非法 blob 键", nameof(key));
+        return full;
+    }
+}