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/TestEnv.cs

@@ -30,9 +30,11 @@ public sealed class TestChannel(Id16 id, ChannelMode mode, byte[] read, byte[] w
     public byte[] WriteToken { get; } = write;
     public byte[] OwnerToken { get; } = owner;
 
+    /// <summary>测试里用 endpoint 当作「哪一个人」:同一个 endpoint 对应同一个账号。</summary>
+    public string AccountOf(byte[] endpoint) => endpoint.AsSpan().SequenceEqual(ownerEndpoint) ? "owner" : "u" + Convert.ToHexString(endpoint)[..8].ToLowerInvariant();
     public Credentials Owner(byte[] endpoint) => new(Role.Owner, OwnerToken, endpoint, "owner");
-    public Credentials Member(byte[] endpoint) => new(Role.Write, WriteToken, endpoint, endpoint.AsSpan().SequenceEqual(ownerEndpoint) ? "owner" : null);
-    public Credentials Reader(byte[] endpoint) => new(Role.Read, ReadToken, endpoint, endpoint.AsSpan().SequenceEqual(ownerEndpoint) ? "owner" : null);
+    public Credentials Member(byte[] endpoint) => new(Role.Write, WriteToken, endpoint, AccountOf(endpoint));
+    public Credentials Reader(byte[] endpoint) => new(Role.Read, ReadToken, endpoint, AccountOf(endpoint));
 }
 
 public sealed class TestEnv : IDisposable
@@ -78,7 +80,7 @@ public sealed class TestEnv : IDisposable
             ReadHash = mode == ChannelMode.Enc ? Codec.Encode(TokenHash.Compute(read)) : null,
             WriteHash = withWriteHash ? Codec.Encode(TokenHash.Compute(write)) : null,
             OwnerHash = Codec.Encode(TokenHash.Compute(owner)),
-        }, ownerEndpoint, "owner");
+        }, "owner");
 
         return new TestChannel(id, mode, read, write, owner, ownerEndpoint);
     }