feat(backend): per-tier, per-kind active-game limits with a guest funnel

Cap a player's simultaneous unfinished games per kind (vs_ai, random,
friends) with independent guest and durable-account tiers, held in a new
single-row backend.config table (-1 = unlimited) behind an in-memory cache
and editable live in the admin console (/_gm/limits). Each game is tagged
with games.game_kind on creation.

This replaces the earlier flat MaxActiveQuickGames=10 combined cap: the
per-tier/kind config is the single mechanism, enforced at the same handler
gate (ensureUnderGameLimit by kind on lobby/enqueue) plus the durable
friends cap in CreateInvitation. game.Service.AtGameLimit only resolves the
tier and counts; the limit policy stays at the request edge.

Guests are now refused friend requests, friend-code redemption,
befriend-in-game and invitation creation outright (403 guest_forbidden) --
previously only the UI hid these.

Admin: a kind column in both game lists and the config editor.

Defaults: guest 1 vs_ai / 1 random / 0 friends; durable 10 / 10 / 10.

backend/internal/social/friendcodes.go

@@ -65,6 +65,12 @@ func (svc *Service) IssueFriendCode(ctx context.Context, accountID uuid.UUID) (F
 // ErrRequestBlocked (a block stands between the pair). A redeem bypasses any prior
 // decline between the two: it clears the old row and writes a fresh friendship.
 func (svc *Service) RedeemFriendCode(ctx context.Context, redeemerID uuid.UUID, code string) (uuid.UUID, error) {
+	// A guest cannot use friends: a durable-account feature (the UI hides it).
+	if acc, err := svc.accounts.GetByID(ctx, redeemerID); err != nil {
+		return uuid.UUID{}, err
+	} else if acc.IsGuest {
+		return uuid.UUID{}, ErrGuestForbidden
+	}
 	issuerID, codeID, err := svc.store.liveFriendCodeByHash(ctx, hashFriendCode(code), svc.now())
 	if err != nil {
 		return uuid.UUID{}, err