feat(server)!: require a logged-in account for every channel request

Drop unauthenticated guests from channels: the allow_guests setting, the
/guests endpoints and the guest seat table are gone, and requests without an
account session get 401 account_required. Share links are unaffected.
Contract version is now 8.0.0.

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

@@ -57,15 +57,15 @@ public sealed class WorkspaceStructureTests
         Assert.Equal("新频道", added.Channels.Single().Name); Assert.Equal(1, added.Revision);
     }
     [Fact]
-    public async Task AllowedGuestsCanPlaceTheirUploadsButCannotCreateBusinessStructure()
+    public async Task MembersCanPlaceTheirUploadsButCannotCreateBusinessStructure()
     {
-        using var env = new TestEnv(); var ownerEp = TestEnv.Endpoint(); var guestEp = TestEnv.Endpoint(); var ch = env.Create(ownerEp);
-        var guest = ch.Member(guestEp); env.Service.Join(ch.Id, guest);
+        using var env = new TestEnv(); var ownerEp = TestEnv.Endpoint(); var memberEp = TestEnv.Endpoint(); var ch = env.Create(ownerEp);
+        var guest = ch.Member(memberEp); env.Service.Join(ch.Id, guest);
         var item = TestEnv.NewId();
         await env.Service.AppendFrameAsync(ch.Id, guest, TestEnv.Frame(item, 0, TestEnv.SmallEnc, TestEnv.EncPayload), null);
         var channelId = env.Service.GetWorkspace(ch.Id, ch.Owner(ownerEp)).Channels.Single().Id;
         Assert.Single(env.Service.MoveWorkspaceItems(ch.Id, guest, [item.ToBase64Url()], channelId, null).Locations);
-        Assert.Equal(ErrorCode.AccountRequired, Assert.Throws<WebDropException>(() => env.Service.CreateWorkspaceGroup(ch.Id, guest, "访客分组")).Code);
+        Assert.Equal(ErrorCode.Forbidden, Assert.Throws<WebDropException>(() => env.Service.CreateWorkspaceGroup(ch.Id, guest, "成员分组")).Code);
     }
 
 }