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/store.go

@@ -200,22 +200,27 @@ func openMatchKey(variant string, multipleWords bool) int64 {
 // order) used only when a game is created; callerName is the caller's display-name
 // snapshot, stamped on their seat whether they open a fresh game or fill another
 // player's open one.
-func (s *Store) OpenOrJoin(ctx context.Context, accountID uuid.UUID, callerName string, ins gameInsert, seats []seatInsert) (gameID uuid.UUID, joined, created bool, err error) {
+func (s *Store) OpenOrJoin(ctx context.Context, accountID uuid.UUID, callerName string, ins gameInsert, seats []seatInsert, exclude []uuid.UUID) (gameID uuid.UUID, joined, created bool, err error) {
 	err = withTx(ctx, s.db, func(tx *sql.Tx) error {
 		if _, e := tx.ExecContext(ctx, `SELECT pg_advisory_xact_lock($1)`,
 			openMatchKey(ins.variant, ins.multipleWordsPerTurn)); e != nil {
 			return fmt.Errorf("open match lock: %w", e)
 		}
 		// 1. Another player's open game waiting for an opponent — fill its seat and start it.
+		// A game whose waiting player is in exclude (the caller's per-user block set, either
+		// direction) is skipped, so a block keeps the pair out of the same anonymous game;
+		// an empty exclude (the "{}" literal) excludes nothing.
 		var other uuid.UUID
 		switch e := tx.QueryRowContext(ctx,
 			`SELECT g.game_id FROM backend.games g
 			  WHERE g.status = 'open' AND g.variant = $1 AND g.multiple_words_per_turn = $2
 			    AND NOT EXISTS (SELECT 1 FROM backend.game_players p
 			                     WHERE p.game_id = g.game_id AND p.account_id = $3)
+			    AND NOT EXISTS (SELECT 1 FROM backend.game_players b
+			                     WHERE b.game_id = g.game_id AND b.account_id = ANY($4::uuid[]))
 			  ORDER BY g.created_at
 			  LIMIT 1 FOR UPDATE SKIP LOCKED`,
-			ins.variant, ins.multipleWordsPerTurn, accountID).Scan(&other); {
+			ins.variant, ins.multipleWordsPerTurn, accountID, uuidArrayLiteral(exclude)).Scan(&other); {
 		case e == nil:
 			if er := fillOpenSeat(ctx, tx, other, accountID, callerName); er != nil {
 				return er