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.

ui/src/lib/codec.test.ts

@@ -237,6 +237,7 @@ describe('codec', () => {
     fb.GameView.addSeats(b, seats);
     fb.GameView.addLastActivityUnix(b, BigInt(1717000000));
     fb.GameView.addVsAi(b, true);
+    fb.GameView.addUnreadChat(b, true);
     const game = fb.GameView.endGameView(b);
     const games = fb.GameList.createGamesVector(b, [game]);
     fb.GameList.startGameList(b);
@@ -251,6 +252,7 @@ describe('codec', () => {
     expect(gl.games[0].seats[0].score).toBe(13);
     expect(gl.games[0].lastActivityUnix).toBe(1717000000);
     expect(gl.games[0].vsAi).toBe(true);
+    expect(gl.games[0].unreadChat).toBe(true);
     expect(gl.atGameLimit).toBe(true);
   });