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.

backend/internal/adminconsole/templates/pages/messages.gohtml

@@ -6,6 +6,7 @@
 {{if .UserID}}<input type="hidden" name="user" value="{{.UserID}}">{{end}}
 <input name="name" value="{{.NameMask}}" placeholder="sender name mask (* ?)">
 <input name="ext" value="{{.ExtMask}}" placeholder="sender external id mask (* ?)">
+<label class="check"><input type="checkbox" name="unread" value="1"{{if .UnreadOnly}} checked{{end}}> unread only</label>
 <button type="submit">Filter</button>
 <a class="export" href="/_gm/messages.csv?{{.FilterQuery}}">Export CSV ↓</a>
 </form>
@@ -13,19 +14,20 @@
 <p class="note">Filtered{{if .GameID}} to game <a href="/_gm/games/{{.GameID}}">{{.GameID}}</a>{{end}}{{if .UserID}} from <a href="/_gm/users/{{.UserID}}">sender</a>{{end}} · <a href="/_gm/messages">clear</a></p>
 {{end}}
 <table class="list">
-<thead><tr><th>Time</th><th>Source</th><th>Sender</th><th>IP</th><th>Message</th><th>Game</th></tr></thead>
+<thead><tr><th>Time</th><th>Source</th><th>Sender</th><th>IP</th><th>Message</th><th>Read</th><th>Game</th></tr></thead>
 <tbody>
 {{range .Items}}
 <tr>
-<td>{{.CreatedAt}}</td>
+<td><a href="/_gm/messages/{{.ID}}">{{.CreatedAt}}</a></td>
 <td>{{.Source}}</td>
 <td><a href="/_gm/users/{{.SenderID}}">{{.SenderName}}</a></td>
 <td>{{.IP}}</td>
 <td>{{.Body}}</td>
+<td>{{if .Unread}}unread{{else}}read{{end}}</td>
 <td><a href="/_gm/games/{{.GameID}}">game</a></td>
 </tr>
 {{else}}
-<tr><td colspan="6"><span class="note">no messages</span></td></tr>
+<tr><td colspan="7"><span class="note">no messages</span></td></tr>
 {{end}}
 </tbody>
 </table>