feat(files): file service owns file domains, quotas and S3 backup

- file domains are created, queried, sized and deleted by the file service
  (POST/GET/DELETE /file/v1/domains), with per-domain quota enforced on write
- /file/v1/options publishes the quota choices; nothing about files lives on
  the host any more
- tickets carry the issuing space and a unique id: a domain only accepts tickets
  from its own space, and read/delete tickets work once (no replaying a counted
  download)
- uploads stay pending until the client confirms them; abandoned ones are swept
  after 24h, and an upload ticket can cancel its own bytes
- backup target is S3 (SigV4 client, no SDK) instead of WebDAV
- CORS allows the methods the browser really uses (POST, DELETE) and exposes Retry-After

server/tests/WebDrop.Api.Tests/FakeS3.cs

@@ -0,0 +1,89 @@
+using System.Collections.Concurrent;
+using System.Security.Cryptography;
+using System.Text;
+using System.Xml.Linq;
+using Microsoft.AspNetCore.Builder;
+using Microsoft.AspNetCore.Hosting.Server;
+using Microsoft.AspNetCore.Hosting.Server.Features;
+using Microsoft.AspNetCore.Http;
+using Microsoft.Extensions.DependencyInjection;
+using Microsoft.Extensions.Logging;
+
+namespace WebDrop.Api.Tests;
+
+/// <summary>
+/// 只实现备份用到的 S3 子集(PUT、HEAD、DELETE、ListObjectsV2),路径风格,数据放在内存里。
+/// 检查 SigV4 的凭据字段和 x-amz-content-sha256 与正文是否一致;不重算签名本身(签名算法由真实服务兜底)。
+/// </summary>
+public sealed class FakeS3 : IAsyncDisposable
+{
+    private readonly WebApplication _app;
+    /// <summary>键(不含存储桶)→ 内容</summary>
+    public ConcurrentDictionary<string, byte[]> Files { get; } = new();
+    public bool Down { get; set; }
+    public string Url { get; private set; } = "";
+    public const string AccessKey = "AKIATEST", SecretKey = "secret", Bucket = "bk";
+
+    public FakeS3()
+    {
+        var builder = WebApplication.CreateSlimBuilder();
+        builder.Configuration["urls"] = "http://127.0.0.1:0";
+        builder.Logging.ClearProviders();
+        _app = builder.Build();
+        _app.Map("/{bucket}/{**key}", async (HttpContext ctx, string bucket, string? key) =>
+        {
+            if (Down) { ctx.Response.StatusCode = 503; return; }
+            string auth = ctx.Request.Headers.Authorization.ToString();
+            if (bucket != Bucket || !auth.StartsWith($"AWS4-HMAC-SHA256 Credential={AccessKey}/", StringComparison.Ordinal) || !auth.Contains("Signature=") || !ctx.Request.Headers.ContainsKey("x-amz-date"))
+            { ctx.Response.StatusCode = 403; return; }
+            key = Uri.UnescapeDataString(key ?? "");
+            switch (ctx.Request.Method)
+            {
+                case "PUT":
+                    {
+                        using var ms = new MemoryStream();
+                        await ctx.Request.Body.CopyToAsync(ms);
+                        var body = ms.ToArray();
+                        if (!string.Equals(ctx.Request.Headers["x-amz-content-sha256"], Convert.ToHexStringLower(SHA256.HashData(body)), StringComparison.Ordinal)) { ctx.Response.StatusCode = 400; return; }
+                        Files[key] = body;
+                        ctx.Response.StatusCode = 200;
+                        break;
+                    }
+                case "HEAD": ctx.Response.StatusCode = Files.ContainsKey(key) ? 200 : 404; break;
+                case "DELETE": Files.TryRemove(key, out _); ctx.Response.StatusCode = 204; break;
+                case "GET":
+                    {
+                        string prefix = ctx.Request.Query["prefix"].ToString(), delimiter = ctx.Request.Query["delimiter"].ToString();
+                        var keys = Files.Keys.Where(k => k.StartsWith(prefix, StringComparison.Ordinal)).Order(StringComparer.Ordinal).ToList();
+                        XNamespace ns = "http://s3.amazonaws.com/doc/2006-03-01/";
+                        var root = new XElement(ns + "ListBucketResult", new XElement(ns + "IsTruncated", "false"));
+                        var folders = new SortedSet<string>(StringComparer.Ordinal);
+                        foreach (var k in keys)
+                        {
+                            int cut = delimiter.Length == 0 ? -1 : k.IndexOf(delimiter, prefix.Length, StringComparison.Ordinal);
+                            if (cut < 0) root.Add(new XElement(ns + "Contents", new XElement(ns + "Key", k)));
+                            else folders.Add(k[..(cut + delimiter.Length)]);
+                        }
+                        foreach (var f in folders) root.Add(new XElement(ns + "CommonPrefixes", new XElement(ns + "Prefix", f)));
+                        ctx.Response.StatusCode = 200;
+                        ctx.Response.ContentType = "application/xml";
+                        await ctx.Response.WriteAsync(root.ToString(), Encoding.UTF8);
+                        break;
+                    }
+                default: ctx.Response.StatusCode = 405; break;
+            }
+        });
+    }
+
+    public async Task StartAsync()
+    {
+        await _app.StartAsync();
+        Url = _app.Services.GetRequiredService<IServer>().Features.Get<IServerAddressesFeature>()!.Addresses.First().TrimEnd('/');
+    }
+
+    public async ValueTask DisposeAsync()
+    {
+        await _app.StopAsync();
+        await _app.DisposeAsync();
+    }
+}