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/engine/engine.go

@@ -47,6 +47,16 @@ func (v Variant) String() string {
 	return "unknown"
 }
 
+// Language returns the variant's interface/bot language tag: "en" for English Scrabble, "ru" for
+// the Russian variants (Russian Scrabble and Erudite). It routes a game's out-of-app push to the
+// matching per-language Telegram bot — by the game, not the recipient's last-login bot (Stage 17).
+func (v Variant) Language() string {
+	if v == VariantEnglish {
+		return "en"
+	}
+	return "ru"
+}
+
 // ruleset returns the scrabble-solver ruleset backing the variant and true, or
 // (nil, false) for an unrecognised variant.
 func (v Variant) ruleset() (*rules.Ruleset, bool) {