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.

composer.json

@@ -9,6 +9,8 @@
         "php": ">=8.5",
         "ext-ctype": "*",
         "ext-iconv": "*",
+        "ext-intl": "*",
+        "ext-zip": "*",
         "doctrine/dbal": "^4.4.3",
         "doctrine/doctrine-bundle": "^3.2.2",
         "doctrine/doctrine-migrations-bundle": "^4.0",
@@ -34,6 +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.*",