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/security.yaml

@@ -27,6 +27,8 @@ security:
             remember_me:
                 secret: '%kernel.secret%'
                 lifetime: 604800 # 1 week in seconds
+            login_throttling:
+                max_attempts: 5
 
     access_control:
         - { path: ^/admin, roles: ROLE_ADMIN }
@@ -43,3 +45,7 @@ when@test:
                 cost: 4 # Lowest possible value for bcrypt
                 time_cost: 3 # Lowest possible value for argon
                 memory_cost: 10 # Lowest possible value for argon
+        firewalls:
+            main:
+                login_throttling:
+                    max_attempts: 2