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/lobby/invitations.go

@@ -150,6 +150,14 @@ func (svc *InvitationService) emitInvitationUpdate(ctx context.Context, inv Invi
 	}
 	intents := make([]notify.Intent, 0, len(recipients))
 	for _, id := range recipients {
+		// An invitee who has blocked the inviter never sees this invitation, so they must not
+		// receive its updates either (an update would otherwise re-add it to their lobby). The
+		// inviter always gets updates; on a lookup error suppress rather than risk a leak.
+		if id != inv.InviterID {
+			if blk, err := svc.blocker.Blocks(ctx, id, inv.InviterID); err != nil || blk {
+				continue
+			}
+		}
 		intents = append(intents, notify.NotificationInvitationUpdate(id, summary))
 	}
 	svc.pub.Publish(intents...)
@@ -211,6 +219,11 @@ func (svc *InvitationService) CreateInvitation(ctx context.Context, inviterID uu
 		return Invitation{}, fmt.Errorf("%w: turn timeout %s not allowed", ErrInvalidInvitation, settings.TurnTimeout)
 	}
 	seen := map[uuid.UUID]bool{inviterID: true}
+	// suppressed collects invitees who have blocked the inviter: the invitation is still
+	// created and persisted for them, but they are never notified and never see it (their
+	// friendship with the inviter survived the block, so they can still appear in the
+	// inviter's picker — they must not learn of the block).
+	suppressed := make(map[uuid.UUID]bool)
 	for _, id := range inviteeIDs {
 		if seen[id] {
 			return Invitation{}, fmt.Errorf("%w: %s invited twice or is the inviter", ErrInvalidInvitation, id)
@@ -222,13 +235,22 @@ func (svc *InvitationService) CreateInvitation(ctx context.Context, inviterID uu
 			}
 			return Invitation{}, err
 		}
-		blocked, err := svc.blocker.IsBlocked(ctx, inviterID, id)
+		// A block the inviter placed refuses the invite (the inviter is aware; the picker hides
+		// the invitee anyway). A block the invitee placed on the inviter is instead suppressed.
+		iBlock, err := svc.blocker.Blocks(ctx, inviterID, id)
 		if err != nil {
 			return Invitation{}, err
 		}
-		if blocked {
+		if iBlock {
 			return Invitation{}, ErrInvitationBlocked
 		}
+		theyBlock, err := svc.blocker.Blocks(ctx, id, inviterID)
+		if err != nil {
+			return Invitation{}, err
+		}
+		if theyBlock {
+			suppressed[id] = true
+		}
 	}
 
 	id, err := uuid.NewV7()
@@ -253,7 +275,18 @@ func (svc *InvitationService) CreateInvitation(ctx context.Context, inviterID uu
 	if err != nil {
 		return Invitation{}, err
 	}
-	svc.emitInvitation(ctx, inv, inviteeIDs)
+	// Notify every invitee except those who have blocked the inviter (their copy stays
+	// stored but unseen — the store-but-hide that keeps the block invisible).
+	notifyIDs := inviteeIDs
+	if len(suppressed) > 0 {
+		notifyIDs = make([]uuid.UUID, 0, len(inviteeIDs))
+		for _, id := range inviteeIDs {
+			if !suppressed[id] {
+				notifyIDs = append(notifyIDs, id)
+			}
+		}
+	}
+	svc.emitInvitation(ctx, inv, notifyIDs)
 	return inv, nil
 }
 
@@ -347,6 +380,17 @@ func (svc *InvitationService) ListInvitations(ctx context.Context, accountID uui
 		if err != nil {
 			return nil, err
 		}
+		// Hide an invitation whose inviter the viewer has blocked: it stays stored but must
+		// never surface to the blocker. The viewer's own invitations (as inviter) are unaffected.
+		if inv.InviterID != accountID {
+			blocked, err := svc.blocker.Blocks(ctx, accountID, inv.InviterID)
+			if err != nil {
+				return nil, err
+			}
+			if blocked {
+				continue
+			}
+		}
 		out = append(out, inv)
 	}
 	return out, nil