Add translation CRUD routes and controller

POST/DELETE for translations gated by can:update,document middleware.
Locale validated against SUPPORTED_LOCALES. Default-locale deletion
returns 422; duplicate-locale add returns 422. Flash messages added
to en/ja lang files (other locales updated in Task 9).

src/lang/en/messages.php

@@ -39,6 +39,8 @@
         'content_label' => 'Content',
         'content_placeholder' => 'Write your markdown here...',
         'saving' => 'Saving...',
+        'translation_added' => 'Translation added.',
+        'translation_deleted' => 'Translation deleted.',
     ],
 
     // Quick Switcher