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/ChatMessageEntity.cs

@@ -0,0 +1,39 @@
+using System.ComponentModel.DataAnnotations;
+
+namespace DorkNet.Server.Data.Entities;
+
+/// <summary>
+/// In-game text-chat message. Sent to the chat.rec.net/thread
+/// endpoint family; backed by a flat per-thread row table where the
+/// thread id is just the canonical (lower, higher) pair of player ids
+/// for DMs, or the room id for room-wide chats.
+///
+/// We keep a separate table from <see cref="MessageEntity"/> because
+/// the wire shape and history-fetch semantics differ — chat is
+/// append-and-read-recent, messages are inbox-style with read
+/// receipts.
+/// </summary>
+public class ChatMessageEntity
+{
+    public long Id { get; set; }
+
+    /// <summary>String thread key. Format: <c>dm:<low>:<high></c> for
+    /// direct messages between two players (lower id first), or
+    /// <c>room:<roomId></c> for room-wide chats.</summary>
+    [MaxLength(64)]
+    public string ThreadKey { get; set; } = string.Empty;
+
+    public long SenderPlayerId { get; set; }
+
+    [MaxLength(2000)]
+    public string Body { get; set; } = string.Empty;
+
+    public DateTime SentAt { get; set; } = DateTime.UtcNow;
+
+    /// <summary>Build the canonical DM thread key for two players.
+    /// Order-independent so both sides hit the same row.</summary>
+    public static string DmKey(long a, long b) =>
+        a < b ? $"dm:{a}:{b}" : $"dm:{b}:{a}";
+
+    public static string RoomKey(long roomId) => $"room:{roomId}";
+}