feat(files): store files on the host when a channel has no file domain

Chat stays usable when the file service is off or down. Files up to 256 MB are hosted like chat; downloads try the host first and fall back to file-service tickets.

Co-Authored-By: Claude Sonnet 5.5 <noreply@anthropic.com> [CI]

server/tests/WebDrop.Core.Tests/ChannelCryptoTests.cs

@@ -86,6 +86,7 @@ public sealed class ChannelCryptoTests
         using var env = new TestEnv(); var ownerEndpoint = TestEnv.Endpoint(); var memberEndpoint = TestEnv.Endpoint();
         var space = env.Create(ownerEndpoint, ChannelMode.Raw); var owner = space.Owner(ownerEndpoint);
         var channelId = env.Service.GetWorkspace(space.Id, owner).Channels.Single().Id;
+        env.Service.SetWorkspaceChannelFileDomain(space.Id, owner, channelId, "domain-1");
         var oldFile = TestEnv.NewId();
         env.Service.DeclareObject(space.Id, owner, new ObjectEnvelope { ItemId = oldFile.ToBase64Url(), ChannelId = channelId, Domain = CryptoDomain.Files, Epoch = 0, KeyEnvelope = "" });
         await Register(env.Service, space.Id, owner, TestEnv.Frame(oldFile, 0, FrameFlags.Final, 16));

server/tests/WebDrop.Core.Tests/ChannelServiceTests.cs

@@ -167,7 +167,16 @@ public class ChannelServiceTests : IDisposable
         var item = TestEnv.NewId();
         Svc.DeclareObject(space.Id, owner, new ObjectEnvelope { ItemId = item.ToBase64Url(), ChannelId = channelId, Domain = CryptoDomain.Files, Epoch = 0, KeyEnvelope = "" });
 
-        // 文件域的对象不能把字节直接交给主后端
+        // 频道没有文件域(文件服务没开、小文件):文件和聊天一样存在主后端,不能整个登记
+        var hosted = TestEnv.NewId();
+        Svc.DeclareObject(space.Id, owner, new ObjectEnvelope { ItemId = hosted.ToBase64Url(), ChannelId = channelId, Domain = CryptoDomain.Files, Epoch = 0, KeyEnvelope = "" });
+        Assert.True((await Svc.AppendFrameAsync(space.Id, owner, TestEnv.Frame(hosted, 0, FrameFlags.Gated | FrameFlags.Final, 60), 3)).Created);
+        var hostedFrame = TestEnv.Frame(hosted, 0, FrameFlags.Gated | FrameFlags.Final, 60);
+        Assert.Equal(ErrorCode.BadRequest, Assert.Throws<WebDropException>(() => Svc.RegisterFileItem(space.Id, owner, hostedFrame.AsMemory(0, Envelope.HeaderSize),
+            [new ChannelService.FrameEntry(60, System.Security.Cryptography.SHA256.HashData(hostedFrame))], 3)).Code);
+
+        // 分配了文件域的频道:文件域的对象不能把字节直接交给主后端
+        Svc.SetWorkspaceChannelFileDomain(space.Id, owner, channelId, "domain-1");
         byte[] first = TestEnv.Frame(item, 0, FrameFlags.Gated, 120);
         await AssertCode(ErrorCode.UseFileApi, () => Svc.AppendFrameAsync(space.Id, owner, first, null));