Stage 17: fix the robot-nudge frequency + per-game push language

Two owner-reported defects from a live contour game.

A. Frequency: the robot's proactive nudge fired hourly for 12h+ (a 12h idle threshold
   then the 1h cooldown, uncapped). Replaced with a lengthening, randomized schedule
   (proactiveNudgeGap): the first nudge ~60-90 min into the human's turn, each later gap
   growing toward 1-6h (uniform sample in [60min, ceil], ceil ramping 90min->6h over 12h
   of idle, measured from the previous nudge), so a long wait gets a handful of
   increasingly-spaced reminders instead of a stream.

B. Language: out-of-app push routed by the recipient's GLOBAL service_language
   (last-login-wins), so after re-logging via the RU bot an English game's nudges came
   from the RU bot. Now a game push (your_turn, game_over, nudge, match_found) carries
   the game's own language (engine.Variant.Language) on push.Event, and the gateway
   routes by it (falling back to service_language for non-game pushes). The New-Game
   variant-gating guarantees the game's bot is one the player has started, so delivery is
   never blocked.

Tests: proactiveNudgeGap unit + retimed TestRobotProactiveNudge; TestVariantLanguage;
emit your_turn/game_over language; TestNudgeRoutedByGameLanguage integration. Docs:
ARCHITECTURE (§7 nudge, §10/§13 routing), FUNCTIONAL (+ _ru), PLAN tracker.

backend/internal/pushgrpc/server.go

@@ -54,10 +54,11 @@ func (s *Service) Subscribe(req *pushv1.SubscribeRequest, stream grpc.ServerStre
 				return nil
 			}
 			ev := &pushv1.Event{
-				UserId:  in.UserID.String(),
-				Kind:    in.Kind,
-				Payload: in.Payload,
-				EventId: in.EventID,
+				UserId:   in.UserID.String(),
+				Kind:     in.Kind,
+				Payload:  in.Payload,
+				EventId:  in.EventID,
+				Language: in.Language,
 			}
 			if err := stream.Send(ev); err != nil {
 				return err