perf(store): index (Receiver, Date) so tell history streams sorted

GetTellHistoryWithSender filters on Receiver and orders by Date DESC. Without a
matching index SQLite sorted the whole receiver history into a temp b-tree
before yielding row one -- measured 10 ms to first row against 9621 tells, all
of it under TabsListLock, which defeats the early break in the caller.

(Receiver, ChatType, Date) does not help: the ChatType IN filter sits between
the equality prefix and the sort column, so the temp b-tree stays. Verified on
a real database: plan now reads SEARCH ... USING INDEX idx_messages_receiver_date
with no sort step.

No SQL LIMIT -- an earlier 500-row cap was removed in v1.4.10 because it cut
less-frequent partners off the back of the window, and 83% of partners have
fewer than 21 tells in total.

The migration dispatcher is cumulative, so Migrate5 is appended to every
existing case, not just the new one.

HellionChat/MessageStore.cs

@@ -279,18 +279,25 @@ internal class MessageStore : IDisposable
                 migrationsToDo.Add(Migrate2);
                 migrationsToDo.Add(Migrate3);
                 migrationsToDo.Add(Migrate4);
+                migrationsToDo.Add(Migrate5);
                 break;
             case 1:
                 migrationsToDo.Add(Migrate2);
                 migrationsToDo.Add(Migrate3);
                 migrationsToDo.Add(Migrate4);
+                migrationsToDo.Add(Migrate5);
                 break;
             case 2:
                 migrationsToDo.Add(Migrate3);
                 migrationsToDo.Add(Migrate4);
+                migrationsToDo.Add(Migrate5);
                 break;
             case 3:
                 migrationsToDo.Add(Migrate4);
+                migrationsToDo.Add(Migrate5);
+                break;
+            case 4:
+                migrationsToDo.Add(Migrate5);
                 break;
         }
 
@@ -430,6 +437,23 @@ internal class MessageStore : IDisposable
         SetMigrationVersion(4);
     }
 
+    private void Migrate5()
+    {
+        _logger.LogInformation("Running migration 5: Add (Receiver, Date) index for tell history");
+
+        // GetTellHistoryWithSender filters on Receiver and orders by Date DESC.
+        // Without a matching index SQLite sorts the whole receiver history into a
+        // temp b-tree before returning row one, which defeats the early break in
+        // the caller. (Receiver, ChatType, Date) does NOT help: the ChatType IN
+        // filter sits between the equality prefix and the sort column.
+        using var cmd = Connection.CreateCommand();
+        cmd.CommandText =
+            "CREATE INDEX IF NOT EXISTS idx_messages_receiver_date ON messages (Receiver, Date);";
+        cmd.ExecuteNonQuery();
+
+        SetMigrationVersion(5);
+    }
+
     private void SetMigrationVersion(int version)
     {
         _logger.LogInformation($"Setting version {version}");
@@ -1014,7 +1038,8 @@ internal class MessageStore : IDisposable
     }
 
     // Returns up to `limit` tells exchanged with the named player, oldest-first.
-    // SQL narrows by Receiver + ChatType via the (Receiver, Date) index, then
+    // SQL narrows by Receiver + ChatType via the (Receiver, Date) index (migration
+    // 5; before that the ordering fell back to a temp b-tree over all rows), then
     // the client-side loop runs PlayerPayload comparison and breaks once
     // `limit` partner matches accumulate. Earlier versions had a hardcoded
     // 500-row scan cap that cut less-frequent pinned partners off the back of