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/packages/framework.yaml

@@ -1,7 +1,7 @@
 # see https://symfony.com/doc/current/reference/configuration/framework.html
 framework:
     secret: '%env(APP_SECRET)%'
-    
+
     # Note that the session will be started ONLY if you read or write from it.
     session: true
     form:
@@ -9,6 +9,13 @@ framework:
             enabled: true
         #esi: true
         #fragments: true
+    rate_limiter:
+        # Throttles guesses against the public season-code entry point (config/packages/security.yaml
+        # handles throttling for the backoffice /login form separately).
+        season_code:
+            policy: sliding_window
+            limit: 20
+            interval: '1 minute'
 when@prod:
     framework:
         # shortcut for private IP address ranges of your proxy
@@ -21,3 +28,6 @@ when@test:
         test: true
         session:
             storage_factory_id: session.storage.factory.mock_file
+        rate_limiter:
+            season_code:
+                limit: 3