fix: selective leaderboard cache invalidation instead of full clear (BUG-S10)

Only invalidates __alltime__ and current season cache keys on fight completion,
preserving historical season caches. Test verifies selective behavior.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

server/src/engine/orchestrator.ts

@@ -577,8 +577,9 @@ async function executeFightRounds(fightId: string, botA: BotRecord, botB: BotRec
     trackMetric(`challenge_${ct}`)
   }
 
-  // Invalidate leaderboard cache after Elo/stats update
-  invalidateLeaderboardCache()
+  // Invalidate leaderboard cache — selective: only alltime + current season
+  const currentSeason = getCurrentSeason()
+  invalidateLeaderboardCache(currentSeason?.id)
 
   // Advance tournament bracket if this was a tournament match
   try { onTournamentFightFinished(fightId, winnerId ?? null) } catch { /* not a tournament fight */ }