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.lock

@@ -4,7 +4,7 @@
         "Read more about it at https://getcomposer.org/doc/01-basic-usage.md#installing-dependencies",
         "This file is @generated automatically"
     ],
-    "content-hash": "0bb88a3f492fa607203eb66735827179",
+    "content-hash": "8d015cbf63f1f3814ab4992817f899fd",
     "packages": [
         {
             "name": "composer/pcre",
@@ -6985,6 +6985,80 @@
             ],
             "time": "2026-05-29T05:06:50+00:00"
         },
+        {
+            "name": "symfony/rate-limiter",
+            "version": "v8.1.1",
+            "source": {
+                "type": "git",
+                "url": "https://github.com/symfony/rate-limiter.git",
+                "reference": "dd8f48286c000b8511b9413105f4118c8c2a4bd6"
+            },
+            "dist": {
+                "type": "zip",
+                "url": "https://api.github.com/repos/symfony/rate-limiter/zipball/dd8f48286c000b8511b9413105f4118c8c2a4bd6",
+                "reference": "dd8f48286c000b8511b9413105f4118c8c2a4bd6",
+                "shasum": ""
+            },
+            "require": {
+                "php": ">=8.4.1",
+                "symfony/options-resolver": "^7.4|^8.0"
+            },
+            "require-dev": {
+                "psr/cache": "^1.0|^2.0|^3.0",
+                "symfony/lock": "^7.4|^8.0"
+            },
+            "type": "library",
+            "autoload": {
+                "psr-4": {
+                    "Symfony\\Component\\RateLimiter\\": ""
+                },
+                "exclude-from-classmap": [
+                    "/Tests/"
+                ]
+            },
+            "notification-url": "https://packagist.org/downloads/",
+            "license": [
+                "MIT"
+            ],
+            "authors": [
+                {
+                    "name": "Wouter de Jong",
+                    "email": "wouter@wouterj.nl"
+                },
+                {
+                    "name": "Symfony Community",
+                    "homepage": "https://symfony.com/contributors"
+                }
+            ],
+            "description": "Provides a Token Bucket implementation to rate limit input and output in your application",
+            "homepage": "https://symfony.com",
+            "keywords": [
+                "limiter",
+                "rate-limiter"
+            ],
+            "support": {
+                "source": "https://github.com/symfony/rate-limiter/tree/v8.1.1"
+            },
+            "funding": [
+                {
+                    "url": "https://symfony.com/sponsor",
+                    "type": "custom"
+                },
+                {
+                    "url": "https://github.com/fabpot",
+                    "type": "github"
+                },
+                {
+                    "url": "https://github.com/nicolas-grekas",
+                    "type": "github"
+                },
+                {
+                    "url": "https://tidelift.com/funding/github/packagist/symfony/symfony",
+                    "type": "tidelift"
+                }
+            ],
+            "time": "2026-06-09T11:06:24+00:00"
+        },
         {
             "name": "symfony/routing",
             "version": "v8.1.0",
@@ -13758,7 +13832,9 @@
     "platform": {
         "php": ">=8.5",
         "ext-ctype": "*",
-        "ext-iconv": "*"
+        "ext-iconv": "*",
+        "ext-intl": "*",
+        "ext-zip": "*"
     },
     "platform-dev": {},
     "plugin-api-version": "2.9.0"