fix: add rate limiting to /poll endpoint + fix test type errors (BUG-S3)

- Add rateLimit(1_000, 30) middleware to GET /poll endpoint
- Fix Challenge type errors in human-responses test files (missing baseDamage)
- Add rate-limit unit test verifying 429 after exceeding limit

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

server/src/engine/human-responses-ordering.test.ts

@@ -9,7 +9,7 @@ const testChallenge: Challenge = {
   answers: ['2009'],
   scoring: 'factual',
   timeout_ms: 8000,
-  difficulty: 'easy',
+  baseDamage: 20,
   choices: ['2009', '2010', '2008'],
 }
 

server/src/engine/human-responses.test.ts

@@ -9,7 +9,7 @@ const mockChallenge: Challenge = {
   answers: ['2009'],
   scoring: 'factual',
   timeout_ms: 8000,
-  difficulty: 'medium',
+  baseDamage: 20,
   choices: ['2009', '2010', '2008'],
 }