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/game_limit_test.go

@@ -42,7 +42,7 @@ func TestCountActiveQuickGames(t *testing.T) {
 	// An open (awaiting-opponent) quick game counts.
 	if _, _, err := games.OpenOrJoin(ctx, human, game.CreateParams{
 		Variant: engine.VariantEnglish, TurnTimeout: 24 * time.Hour,
-	}, time.Now().Add(time.Minute)); err != nil {
+	}, time.Now().Add(time.Minute), nil); err != nil {
 		t.Fatalf("open quick game: %v", err)
 	}
 	// An active quick game and an honest-AI quick game both count (neither has an invitation row).