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/lobby/matchmaker.go

@@ -76,9 +76,12 @@ func (m *Matchmaker) SetNotifier(p notify.Publisher) {
 // emitMatchFound pushes match_found to every seat of a freshly started game.
 // Emitting to a robot seat is harmless (no client subscription exists for it).
 func (m *Matchmaker) emitMatchFound(g game.Game) {
+	lang := g.Variant.Language() // route the push by the game's language, not the recipient's bot (Stage 17)
 	intents := make([]notify.Intent, 0, len(g.Seats))
 	for _, s := range g.Seats {
-		intents = append(intents, notify.MatchFound(s.AccountID, g.ID))
+		mf := notify.MatchFound(s.AccountID, g.ID)
+		mf.Language = lang
+		intents = append(intents, mf)
 	}
 	m.pub.Publish(intents...)
 }