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/Common/Auth.cs

@@ -0,0 +1,19 @@
+using System.Security.Cryptography;
+
+namespace WebDrop.Core;
+
+/// <summary>服务端只保存令牌的 SHA-256,验证时常量时间比较。</summary>
+public static class TokenHash
+{
+    public const int Size = 32;
+
+    public static byte[] Compute(ReadOnlySpan<byte> token) => SHA256.HashData(token);
+
+    public static bool Matches(ReadOnlySpan<byte> token, byte[]? expectedHash)
+    {
+        if (expectedHash is not { Length: Size }) return false;
+        Span<byte> actual = stackalloc byte[Size];
+        SHA256.HashData(token, actual);
+        return CryptographicOperations.FixedTimeEquals(actual, expectedHash);
+    }
+}