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/game/ChatScreen.svelte

@@ -2,7 +2,7 @@
   import { onMount } from 'svelte';
   import Chat from './Chat.svelte';
   import { gateway } from '../lib/gateway';
-  import { app, handleError, clearChatUnread } from '../lib/app.svelte';
+  import { app, handleError, markChatRead } from '../lib/app.svelte';
   import { canSendChat, sentThisTurn } from '../lib/chatlimit';
   import type { ChatMessage, StateView } from '../lib/model';
 
@@ -50,7 +50,7 @@
   async function refresh() {
     try {
       messages = await gateway.chatList(id);
-      clearChatUnread(id);
+      markChatRead(id);
     } catch {
       /* best-effort */
     }