Import december-2020-12-18 from private repo (scrubbed 2026-05-27)

Targets Rec Room 2020.12.18 client.

Re-import after adding:
- DorkNet.ClientMod/dorknet-clientmod.json.template — config schema
  the launcher's manifest-driven patcher renders into UserData/.
- tools/build-patcher-zip.ps1 — produces the dorknet-clientpatch-{key}.zip
  Release asset the launcher fetches.
- .github/workflows/release.yml — auto-publishes both Release assets
  on v1-december-* tag push.

Scrubbed via maintainer-side scrub-for-public.ps1: same drop rules as
the prior import, plus build-patcher-zip.ps1 added to the tools/
allow-list so it survives the scrub.

DorkNet.Server/Data/Entities/PrivateInstanceInviteeEntity.cs

@@ -0,0 +1,29 @@
+namespace DorkNet.Server.Data.Entities;
+
+/// <summary>
+/// Join row marking that <see cref="PlayerId"/> is on the invite list
+/// for <see cref="PrivateInstanceId"/>. Created when the owner sends an
+/// invite via <c>POST /invite</c> (handled in <c>MessagesController</c>);
+/// checked on <c>POST /goto/instance/{id}</c> by
+/// <c>PrivateInstanceService.CanJoin</c>.
+///
+/// Composite PK on (PrivateInstanceId, PlayerId) — naturally unique;
+/// repeat-invite calls become idempotent no-ops.
+/// </summary>
+public class PrivateInstanceInviteeEntity
+{
+    public long PrivateInstanceId { get; set; }
+    public long PlayerId { get; set; }
+    public DateTime InvitedAt { get; set; } = DateTime.UtcNow;
+
+    /// <summary>The MessageEntity.Id of the latest invite for this
+    /// (instance, player). Lets <c>GoToInvite</c> resolve the
+    /// roomInstanceId even when the watch's accept flow has already
+    /// raced through <c>POST /api/messages/v3/delete</c> and removed
+    /// the underlying message row. Without this column the server
+    /// returned ErrorCode 40 ("invite expired") for any successful
+    /// accept where DELETE landed first — which was the common case
+    /// because the 2020 watch fires DELETE and POST /goto/invite/{id}
+    /// in parallel.</summary>
+    public long? LatestInviteMessageId { get; set; }
+}