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/server/handlers.go

@@ -319,6 +319,8 @@ func statusForError(err error) (int, string) {
 		return http.StatusConflict, "request_declined"
 	case errors.Is(err, social.ErrFriendCodeInvalid):
 		return http.StatusUnprocessableEntity, "friend_code_invalid"
+	case errors.Is(err, social.ErrGuestForbidden), errors.Is(err, lobby.ErrGuestForbidden):
+		return http.StatusForbidden, "guest_forbidden"
 	case errors.Is(err, feedback.ErrGuestForbidden):
 		return http.StatusForbidden, "feedback_guest_forbidden"
 	case errors.Is(err, feedback.ErrBanned):