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/views.go

@@ -94,11 +94,13 @@ type MessagesView struct {
 	ExtMask     string
 	GameID      string
 	UserID      string
+	UnreadOnly  bool
 	FilterQuery template.URL
 }
 
 // MessageRow is one chat message in the moderation list: its sender (linked to the user
-// card), source, IP, body, game (linked to the game card) and time.
+// card), source, IP, body, game (linked to the game card), time, and whether it is still
+// unread by at least one recipient.
 type MessageRow struct {
 	ID         string
 	SenderID   string
@@ -108,6 +110,32 @@ type MessageRow struct {
 	Body       string
 	GameID     string
 	CreatedAt  string
+	Unread     bool
+}
+
+// ChatMessageDetailView is one chat message with its per-seat read breakdown, for the
+// console message card.
+type ChatMessageDetailView struct {
+	ID         string
+	GameID     string
+	SenderID   string
+	SenderName string
+	Source     string
+	Kind       string
+	Body       string
+	IP         string
+	CreatedAt  string
+	Unread     bool
+	Seats      []ChatSeatStatusRow
+}
+
+// ChatSeatStatusRow is one seat's read status on the message card: the seat index, the
+// occupant (linked to the user card) and its role ("sender", "read" or "unread").
+type ChatSeatStatusRow struct {
+	Seat        int
+	AccountID   string
+	DisplayName string
+	Role        string
 }
 
 // UserDetailView is one account with its stats, identities and recent games.