Move turnstile check to api (#3845)

## Description

Re-enables Turnstile verification (was temporarily disabled in v31 to
diagnose intermittent `invalid-input-response` rejections) and moves the
verification call off the game servers.

Game servers no longer hold `TURNSTILE_SECRET_KEY` or hit
`challenges.cloudflare.com` directly. Instead they POST to
`${jwtIssuer}/turnstile` on the api-worker (authenticated with the
existing `apiKey`), which holds the secret and proxies to Cloudflare.
Shrinks blast radius and removes the secret from every game host + GH
Actions workflow.

Response from the api-worker is Zod-validated; null tokens short-circuit
to `rejected` locally.

## Please complete the following:

- [x] I have added screenshots for all UI updates (n/a — server only)
- [x] I process any text displayed to the user through translateText()
(n/a — no user-visible text)
- [ ] I have added relevant tests to the test directory
- [x] I confirm I have thoroughly tested these changes and take full
responsibility for any bugs introduced

## Discord:

evanpelle

tests/util/TestServerConfig.ts

@@ -7,9 +7,6 @@ export class TestServerConfig implements ServerConfig {
   turnstileSiteKey(): string {
     throw new Error("Method not implemented.");
   }
-  turnstileSecretKey(): string {
-    throw new Error("Method not implemented.");
-  }
   apiKey(): string {
     throw new Error("Method not implemented.");
   }