feat: add leaderboard cache, performance benchmarks, and verify SSE cleanup

- Add 30s TTL leaderboard cache with invalidation on fight completion
- Add scoreRound performance benchmark: 1000 rounds in <100ms
- Add checkAnswer performance benchmark: 1000 checks in <50ms
- Add adversarial regex backtracking test for checkAnswer
- Verify SSE cleanup: connections, IP counters, spectator counts, event listeners
  all properly decremented in finally block on disconnect

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

server/src/engine/orchestrator.ts

@@ -17,6 +17,7 @@ import { publishFightResult } from './nostr-publish.js'
 import { getCurrentSeason } from './seasons.js'
 import { onFightFinished as onTournamentFightFinished } from './tournaments.js'
 import { trackFightCompleted, trackBotActive, trackMetric } from './analytics.js'
+import { invalidateLeaderboardCache } from '../routes/bots.js'
 
 const webhookResponseSchema = z.object({
   answer: z.string().nullable().optional(),
@@ -539,6 +540,9 @@ async function executeFightRounds(fightId: string, botA: BotRecord, botB: BotRec
     trackMetric(`challenge_${ct}`)
   }
 
+  // Invalidate leaderboard cache after Elo/stats update
+  invalidateLeaderboardCache()
+
   // Advance tournament bracket if this was a tournament match
   try { onTournamentFightFinished(fightId, winnerId ?? null) } catch { /* not a tournament fight */ }