Add rate limiting to login and season-code entry points

Neither /login nor the public season-code guess form (POST /) had
any throttling, making both an unlimited brute-force/enumeration
oracle. Adds symfony/rate-limiter and enables login_throttling on
the main firewall, plus a dedicated per-IP rate limiter on the
season-code form.

composer.json

@@ -36,6 +36,7 @@
         "symfony/object-mapper": "8.1.*",
         "symfony/property-access": "8.1.*",
         "symfony/property-info": "8.1.*",
+        "symfony/rate-limiter": "8.1.*",
         "symfony/runtime": "8.1.*",
         "symfony/security-bundle": "8.1.*",
         "symfony/security-csrf": "8.1.*",