Add CSRF protection, login throttling, and invite-code rate limiting

The admin panel already checked CSRF tokens on destructive actions,
but the player-facing raw HTML forms (create/join/leave/start
session, toggle ready, lobby chat, feedback) had none - cookie
SameSite=Lax blunts classic cross-site auto-submit attacks but isn't
a substitute for real tokens. Adds matching csrf_token()/
isCsrfTokenValid() checks to all of them.

Also adds login_throttling (5 attempts/15 min) to stop unlimited
password guessing, and a per-user rate limiter (10/min) on the
invite-code join endpoint, since invite codes are only 32-bit and
had no protection against brute-forcing.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

config/packages/framework.yaml

@@ -15,6 +15,12 @@ framework:
         storage_factory_id: session.storage.factory.native
         save_path: '%kernel.project_dir%/var/sessions/%kernel.environment%'
 
+    rate_limiter:
+        invite_code_join:
+            policy: 'sliding_window'
+            limit: 10
+            interval: '1 minute'
+
 when@prod:
     framework:
         session:

config/packages/security.yaml

@@ -22,6 +22,9 @@ security:
                 enable_csrf: true
                 username_parameter: username
                 password_parameter: password
+            login_throttling:
+                max_attempts: 5
+                interval: '15 minutes'
             logout:
                 path: app_logout
                 # where to redirect after logout