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/inttest/open_match_test.go

@@ -49,7 +49,7 @@ func openParams(seed int64) game.CreateParams {
 
 func openGame(t *testing.T, svc *game.Service, starter uuid.UUID, seed int64) game.Game {
 	t.Helper()
-	g, joined, err := svc.OpenOrJoin(context.Background(), starter, openParams(seed), time.Now().Add(time.Minute))
+	g, joined, err := svc.OpenOrJoin(context.Background(), starter, openParams(seed), time.Now().Add(time.Minute), nil)
 	if err != nil {
 		t.Fatalf("open game: %v", err)
 	}
@@ -96,7 +96,7 @@ func TestOpenGameStarterWaitsWhenOpponentMovesFirst(t *testing.T) {
 	clearOpenGames(t)
 	svc := newGameService()
 	starter := provisionAccount(t)
-	g, _, err := svc.OpenOrJoin(ctx, starter, openParams(1), time.Now().Add(time.Minute))
+	g, _, err := svc.OpenOrJoin(ctx, starter, openParams(1), time.Now().Add(time.Minute), nil)
 	if err != nil {
 		t.Fatalf("open: %v", err)
 	}
@@ -128,7 +128,7 @@ func TestOpenGameJoinAfterStarterMoved(t *testing.T) {
 	}
 
 	joiner := provisionAccount(t)
-	g2, joined, err := svc.OpenOrJoin(ctx, joiner, openParams(0), time.Now().Add(time.Minute))
+	g2, joined, err := svc.OpenOrJoin(ctx, joiner, openParams(0), time.Now().Add(time.Minute), nil)
 	if err != nil {
 		t.Fatalf("join: %v", err)
 	}