Declare ext-intl/ext-zip requirements and fix security audit findings (#213)

* Declare ext-intl and ext-zip as explicit composer requirements

DataExportService uses ZipArchive directly and the Dutch-locale
format_datetime Twig filter needs real ext-intl (the polyfill only
supports en), but composer.json only declared ext-ctype/ext-iconv.
Adding them lets `composer check-platform-reqs` catch a missing
extension before a runtime crash.

* Add authorization checks to PrepareEliminationController

index and viewElimination had no IsGranted guard, unlike every other
backoffice/elimination controller, so any authenticated user could
prepare or overwrite another season's elimination screens.

* Prevent formula injection in spreadsheet exports

Answer/candidate text starting with =, +, -, or @ was written to XLSX
exports as a live formula rather than plain text. Since seasons can
have multiple owners, a co-owner could plant a formula that runs (and
can exfiltrate data) when another owner opens the export in Excel.

* 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.

* Add unique constraint to prevent double-submit score inflation

A candidate could submit two concurrent POSTs for the same question
before either committed, since GivenAnswer had no unique constraint
and the "next question" check was subject to a TOCTOU race — each
insert was then counted as a correct answer.

* Address CodeRabbit review findings

Include a Retry-After header on the season-code rate limit, correct
stale line references in the security audit doc, and fix a stray
comma in the reset-password email.

config/reference.php

@@ -629,7 +629,7 @@ use Symfony\Component\Config\Loader\ParamConfigurator as Param;
  *         }>,
  *     },
  *     rate_limiter?: bool|array{ // Rate limiter configuration
- *         enabled?: bool|Param, // Default: false
+ *         enabled?: bool|Param, // Default: true
  *         limiters?: array<string, array{ // Default: []
  *             lock_factory?: scalar|Param|null, // The service ID of the lock factory used by this limiter (or null to disable locking). // Default: "auto"
  *             cache_pool?: scalar|Param|null, // The cache pool to use for storing the current limiter state. // Default: "cache.rate_limiter"