Fix MAX chat identity isolation

server/QMax.Api/Program.cs

@@ -165,6 +165,11 @@ static async Task EnsureCompatibilitySchemaAsync(QMaxDbContext db)
         ON MessageAttachments (MessageId, ExternalId);
         """);
 
+    await db.Database.ExecuteSqlRawAsync("""
+        CREATE INDEX IF NOT EXISTS IX_Messages_ChatId_ExternalId
+        ON Messages (ChatId, ExternalId);
+        """);
+
     await db.Database.ExecuteSqlRawAsync("""
         CREATE TABLE IF NOT EXISTS MessageReactions (
             Id TEXT NOT NULL CONSTRAINT PK_MessageReactions PRIMARY KEY,

server/QMax.Api/Services/MaxBridgeSyncService.cs

@@ -73,20 +73,14 @@ public sealed class MaxBridgeSyncService(
         var createdMessages = new List<(Chat Chat, Message Message)>();
         var updatedMessages = new List<(Chat Chat, Message Message)>();
         var previewPushNotifications = new List<(Chat Chat, Message Message)>();
+        var trackedChatsByExternalId = new Dictionary<string, Chat>(StringComparer.Ordinal);
 
         foreach (var update in updates)
         {
-            var chat = await db.Chats
-                .FirstOrDefaultAsync(x => x.ExternalId == update.ExternalId, cancellationToken);
-
-            if (chat is null && update.ExternalId.StartsWith("dom:", StringComparison.OrdinalIgnoreCase))
+            if (!trackedChatsByExternalId.TryGetValue(update.ExternalId, out var chat))
             {
                 chat = await db.Chats
-                    .FirstOrDefaultAsync(
-                        x => x.Title == update.Title &&
-                             x.ExternalId != null &&
-                             x.ExternalId.StartsWith("dom:"),
-                        cancellationToken);
+                    .FirstOrDefaultAsync(x => x.ExternalId == update.ExternalId, cancellationToken);
             }
 
             var chatWasCreated = chat is null;
@@ -103,6 +97,8 @@ public sealed class MaxBridgeSyncService(
                 changed++;
             }
 
+            trackedChatsByExternalId[update.ExternalId] = chat;
+
             var nextAvatarUrl = string.IsNullOrWhiteSpace(update.AvatarUrl)
                 ? chat.AvatarUrl
                 : update.AvatarUrl;
@@ -156,7 +152,10 @@ public sealed class MaxBridgeSyncService(
 
                 var existing = await db.Messages
                     .Include(x => x.Attachments)
-                    .FirstOrDefaultAsync(x => x.ExternalId == incoming.ExternalId, cancellationToken);
+                    .FirstOrDefaultAsync(
+                        x => x.ChatId == chat.Id &&
+                             x.ExternalId == incoming.ExternalId,
+                        cancellationToken);
                 if (existing is not null)
                 {
                     var merge = await MergeExistingMessageAsync(