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/lobby/invitations.go

@@ -15,6 +15,7 @@ import (
 	"scrabble/backend/internal/account"
 	"scrabble/backend/internal/engine"
 	"scrabble/backend/internal/game"
+	"scrabble/backend/internal/gamelimits"
 	"scrabble/backend/internal/notify"
 	"scrabble/backend/internal/postgres/jet/backend/model"
 	"scrabble/backend/internal/postgres/jet/backend/table"
@@ -218,6 +219,20 @@ func (svc *InvitationService) CreateInvitation(ctx context.Context, inviterID uu
 	if !slices.Contains(game.AllowedTurnTimeouts, settings.TurnTimeout) {
 		return Invitation{}, fmt.Errorf("%w: turn timeout %s not allowed", ErrInvalidInvitation, settings.TurnTimeout)
 	}
+	// A guest cannot invite: friend games are a durable-account feature. The UI hides the flow;
+	// this is the server source of truth.
+	if acc, err := svc.accounts.GetByID(ctx, inviterID); err != nil {
+		return Invitation{}, err
+	} else if acc.IsGuest {
+		return Invitation{}, ErrGuestForbidden
+	}
+	// A durable inviter is still capped: refuse a new friend game once the per-tier friends limit is
+	// reached. The guest branch above short-circuits before here, so this is the durable cap.
+	if atLimit, err := svc.games.AtGameLimit(ctx, inviterID, gamelimits.KindFriends); err != nil {
+		return Invitation{}, err
+	} else if atLimit {
+		return Invitation{}, game.ErrGameLimitReached
+	}
 	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
@@ -336,6 +351,7 @@ func (svc *InvitationService) startGame(ctx context.Context, invitationID uuid.U
 		HintsPerPlayer:       inv.Settings.HintsPerPlayer,
 		DropoutTiles:         inv.Settings.DropoutTiles,
 		MultipleWordsPerTurn: inv.Settings.MultipleWordsPerTurn,
+		Kind:                 gamelimits.KindFriends,
 	})
 	if err != nil {
 		return err