feat(chat): unread read-receipts with lobby/game dot and history-open ack

Persist per-message read state as a chat_messages.unread_seats bitmask
(migration 00008): a text message seeds every recipient seat's bit, a nudge
only the awaited seat's. A seat's bit clears when the player opens the move
history or chat (POST /games/:id/chat/read, sent only when something is
unread), and a nudge additionally clears when its recipient answers by moving
(a wired game NudgeClearer, dependency-inverted so game keeps off social).

UI shows a per-viewer unread dot in the lobby (next to the opponent) and the
game score bar — the unread_chat game-view flag seeds it from authoritative
REST views, live chat/nudge events raise it. Opening the move history counts
as reading (even without entering chat): the 💬 fade-blinks twice and the
client acks. Admin Messages gains an unread-only filter, a read/unread column,
and a per-message card with the per-seat read breakdown. Observability:
chat_read_duration histogram + chat_unread_messages gauge + social tracing.

docs/ARCHITECTURE.md

@@ -514,11 +514,24 @@ disguised robot stays indistinguishable from a person.
   has a **Messages** section that lists posted messages (nudges excluded)
   newest-first with the sender's resolved name, **source** (guest / robot / oldest
   identity kind), IP and game, searchable by sender name / external-id glob masks and
-  pinnable to one game or sender (linked from the game and user cards).
+  pinnable to one game or sender (linked from the game and user cards). It also offers an
+  **unread-only filter** and a read/unread column, and each message has a detail card with
+  the **per-seat read breakdown** (sender / read / unread).
 - **Nudge**: folded into the chat as a `nudge` message kind. The player awaiting
   the opponent may nudge **once per hour per game**; it is not allowed on one's own
   turn. The platform-native delivery runs through the gateway and the platform
   side-service.
+- **Read receipts**: each `chat_messages` row carries an `unread_seats` bitmask — a set
+  bit per recipient seat that has **not** yet read it (the sender's own bit is never set).
+  A text message seeds the bits of every seated recipient; a nudge seeds only the awaited
+  player's. A seat's bit clears when that player **opens the move history or the chat**
+  (`POST /games/:id/chat/read`, which the client sends only when it holds unread, so a
+  history open is not a constant backend call), and a **nudge additionally clears when its
+  recipient answers by moving** (the move path calls a wired `NudgeClearer`). The mask is
+  inverted so "anything unread" is a plain `unread_seats <> 0`, which the per-viewer
+  `unread_chat` game-view flag (seeding the lobby and in-game unread **dot**), the admin
+  unread filter and the unread gauge all use. On each clear the publish-to-read latency is
+  recorded; the read time itself is not retained.
 - **Profile**: `preferred_language` (en/ru, edited in Settings), display name, email
   (confirm-code binding, see §4), **timezone**, the daily **away window** and the
   block toggles — all editable through `account.UpdateProfile`, which validates them:
@@ -551,7 +564,8 @@ disguised robot stays indistinguishable from a person.
   `game_moves` (the move journal), `complaints` and `account_stats`, and the
   social/lobby tables `friendships` (the request/accept graph, its status admitting
   `declined`), `blocks`
-  (per-user blocks), `chat_messages` (per-game chat and nudges), `email_confirmations`
+  (per-user blocks), `chat_messages` (per-game chat and nudges, carrying the per-message
+  `unread_seats` read bitmask), `email_confirmations`
   (pending confirm-codes), `game_invitations` / `game_invitation_invitees`
   (friend-game invitations), `friend_codes` (one-time add-a-friend codes),
   `game_drafts` (a player's in-progress rack order + board composition per
@@ -743,7 +757,9 @@ edits take effect on the next `profile.get` (open/reconnect/foreground), not mid
   whose synthetic timing dominates the tail, so per-human analysis lives in the admin
   console, below); counters `games_started_total`, `games_abandoned_total` (a
   turn-timeout seat drop), `chat_messages_total` (`kind` = message/nudge) and
-  `robot_games_finished_total`; an observable gauge `game_cache_active`; the gateway
+  `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
   `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).