feat(social): asymmetric per-user block, in-game block control, admin lists

Make a per-user block one-directional and non-destructive: the blocker stops
receiving everything from the blocked user (chat, nudge, friend requests,
invitations) and the matchmaker never pairs them, while the blocked user
notices nothing — their sends still persist by the normal rules but are never
delivered or surfaced (born-read). A block no longer deletes the friendship
(an unblock cleanly restores it) and instant-reads any unread the blocked user
had left for the blocker.

- backend: a directional blockExists guard across chat/nudge/friends/invitations
  (store-but-hide for the blocked->blocker direction, refuse blocker->blocked);
  the matchmaker excludes a block-related pair (both directions) from auto-match;
  user_blocked/user_unblocked notifications to the blocker only (in-app only).
- ui: the opponent score card gains a block ✖️ control mirroring add-friend
  (red "Block?" confirm, mutual-hide while confirming, struck name, hidden chat
  composer when blocked); optimistic apply + event confirm + rollback for both.
- admin: the user card gains cross-linked blocks / blocked-by / friends lists.
- docs: FUNCTIONAL(+ru), ARCHITECTURE §10 + decision record, UI_DESIGN, PRERELEASE.

backend/internal/game/service.go

@@ -274,7 +274,7 @@ func (svc *Service) Create(ctx context.Context, params CreateParams) (Game, erro
 // pins it. First-move fairness comes from seating the caller at seat 0 or seat 1
 // (derived from the seed): seated at seat 1, the still-empty seat 0 moves first, so the
 // caller just waits for the opponent. It backs the lobby auto-match enqueue.
-func (svc *Service) OpenOrJoin(ctx context.Context, accountID uuid.UUID, params CreateParams, openDeadline time.Time) (Game, bool, error) {
+func (svc *Service) OpenOrJoin(ctx context.Context, accountID uuid.UUID, params CreateParams, openDeadline time.Time, exclude []uuid.UUID) (Game, bool, error) {
 	acc, err := svc.accounts.GetByID(ctx, accountID)
 	if err != nil {
 		if errors.Is(err, account.ErrNotFound) {
@@ -319,7 +319,7 @@ func (svc *Service) OpenOrJoin(ctx context.Context, accountID uuid.UUID, params
 	if seed&1 == 1 {
 		seats = []seatInsert{{}, caller}
 	}
-	gameID, joined, created, err := svc.store.OpenOrJoin(ctx, accountID, acc.DisplayName, ins, seats)
+	gameID, joined, created, err := svc.store.OpenOrJoin(ctx, accountID, acc.DisplayName, ins, seats, exclude)
 	if err != nil {
 		return Game{}, false, err
 	}