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/game/emit_test.go

@@ -68,6 +68,10 @@ func TestEmitMoveNotifiesActor(t *testing.T) {
 	if got := string(yt.ScoreLine()); got != "13:19" { // seat 1 (recipient) first, then seat 0
 		t.Errorf("your_turn score_line = %q, want 13:19", got)
 	}
+	// Routed out-of-app by the game's language (the default Variant is English).
+	if yourTurn.Language != "en" {
+		t.Errorf("your_turn language = %q, want en", yourTurn.Language)
+	}
 }
 
 // TestEmitMoveAnnouncesGameOver checks the closing move sends a game_over push to every seat,
@@ -102,4 +106,7 @@ func TestEmitMoveAnnouncesGameOver(t *testing.T) {
 	if string(l.Result()) != "lost" || string(l.ScoreLine()) != "95:120" {
 		t.Errorf("loser game_over = %q / %q, want lost / 95:120", l.Result(), l.ScoreLine())
 	}
+	if over[winner].Language != "en" || over[loser].Language != "en" {
+		t.Errorf("game_over languages = %q/%q, want en/en", over[winner].Language, over[loser].Language)
+	}
 }