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/scoring.test.ts

@@ -326,3 +326,23 @@ describe('calculateTier', () => {
     expect(calculateTier(1900, 39)).toBe(5) // below Legend wins
   })
 })
+
+describe('scoreRound performance', () => {
+  it('completes 1000 rounds in under 100ms (<0.1ms each)', () => {
+    const challenge = makeChallenge()
+    const botA = { id: 'a1', name: 'AlphaBot' }
+    const botB = { id: 'b1', name: 'BetaBot' }
+
+    const start = performance.now()
+    for (let i = 0; i < 1000; i++) {
+      scoreRound(
+        challenge, botA, botB,
+        makeResponse('4', 200 + i),
+        makeResponse('banana', 500 + i),
+        null, i % 6, 0,
+      )
+    }
+    const elapsed = performance.now() - start
+    expect(elapsed).toBeLessThan(100) // <0.1ms per call
+  })
+})