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.

src/Helpers/FormulaInjectionSafeValueBinder.php

@@ -0,0 +1,31 @@
+<?php
+
+declare(strict_types=1);
+
+namespace Tvdt\Helpers;
+
+use PhpOffice\PhpSpreadsheet\Cell\Cell;
+use PhpOffice\PhpSpreadsheet\Cell\DefaultValueBinder;
+
+/**
+ * Stores any string that would otherwise be auto-detected as a spreadsheet formula (or that
+ * spreadsheet software re-interprets as one on open/paste) as plain text instead, to prevent
+ * formula injection via user-controlled export data (e.g. a candidate or answer named
+ * `=WEBSERVICE(...)`).
+ */
+final class FormulaInjectionSafeValueBinder extends DefaultValueBinder
+{
+    private const string DANGEROUS_PREFIXES = "=+-@\t\r";
+
+    #[\Override]
+    public function bindValue(Cell $cell, mixed $value): bool
+    {
+        if (\is_string($value) && '' !== $value && str_contains(self::DANGEROUS_PREFIXES, $value[0])) {
+            $cell->setValueExplicit($value);
+
+            return true;
+        }
+
+        return parent::bindValue($cell, $value);
+    }
+}