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/GameSessionEntity.cs

@@ -0,0 +1,36 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace DorkNet.Server.Data.Entities;
+
+/// <summary>
+/// Active game session backing the matchmaking <c>JoinOrCreate</c> flow
+/// in <see cref="DorkNet.Server.Services.GameSessionService"/>. Replaces
+/// the in-process <c>ConcurrentDictionary</c> + atomic counter so:
+/// <list type="bullet">
+/// <item>Session ids are allocated by Postgres bigserial — no
+///     replica-local counter that collides with another replica's
+///     after restart.</item>
+/// <item>A "find a non-full session in this room+region" query
+///     coordinates across all replicas; without it, two players
+///     joining via different replicas could each create their own
+///     fresh session for the same room and end up isolated.</item>
+/// </list>
+///
+/// Sessions are deleted when their PlayerCount hits 0 (handled in the
+/// service, no FK cascade needed). Brief mid-rejoin flicker is fine —
+/// the next JoinOrCreate just creates a new session with a new id.
+/// </summary>
+public class GameSessionEntity
+{
+    /// <summary>bigserial — allocated by Postgres on insert. Crosses
+    /// replicas safely because the sequence is shared.</summary>
+    public long Id { get; set; }
+
+    [MaxLength(64)] public string RoomId { get; set; } = string.Empty;
+    [MaxLength(64)] public string ActivityLevelId { get; set; } = string.Empty;
+    [MaxLength(16)] public string Region { get; set; } = "us";
+    [MaxLength(128)] public string PhotonRoomName { get; set; } = string.Empty;
+    public int MaxCapacity { get; set; } = 8;
+    public int PlayerCount { get; set; } = 0;
+    public DateTime CreatedAt { get; set; } = DateTime.UtcNow;
+}