feat(observability): add the Scrabble — Messages Grafana dashboard

Visualises the chat read-receipt metrics added with the read-receipt feature
(the dashboard was deferred there per the owner): posted rate by kind, the
unread backlog (chat_unread_messages gauge), and the publish-to-read latency
(chat_read_duration — average by kind plus overall p50/p95). Mirrors the
game-domain dashboard's structure; the file provider auto-discovers it.

docs/ARCHITECTURE.md

@@ -761,7 +761,8 @@ edits take effect on the next `profile.get` (open/reconnect/foreground), not mid
   turn-timeout seat drop), `chat_messages_total` (`kind` = message/nudge) and
   `robot_games_finished_total`; a histogram `chat_read_duration` (chat publish-to-read
   latency by `kind`); observable gauges `game_cache_active` and `chat_unread_messages`
-  (chat entries with `unread_seats <> 0`); the gateway
+  (chat entries with `unread_seats <> 0`, both chat metrics surfaced on the **Scrabble —
+  Messages** dashboard); the gateway
   `edge_request_duration` (the UI-perceived roundtrip, by `message_type`/`result`);
   and Go runtime/heap metrics. Game-scoped metrics carry a `variant` attribute
   (scrabble_en/scrabble_ru/erudit_ru).