feat: add module-specific locale support for i18n

Enhance the self-contained module architecture with locale/translation support:

ModuleDefinition changes:
- Add locales_path attribute for module-specific translations
- Add get_locales_dir() helper method
- Include locales in validate_structure() check

i18n module changes (app/utils/i18n.py):
- Add get_module_locale_dirs() to discover module locales
- Update load_translations() to merge module translations with core
- Module translations namespaced under module code (e.g., cms.title)
- Add _deep_merge() helper for nested dictionary merging
- Add _load_json_file() helper for cleaner JSON loading

CMS module locales:
- Add app/modules/cms/locales/ with translations for all 4 languages
- en.json, fr.json, de.json, lb.json with CMS-specific strings
- Covers: pages, page editing, SEO, navigation, publishing, homepage
  sections, media library, themes, actions, and messages

Usage in templates:
  {{ _("cms.title") }}           -> "Content Management" (en)
  {{ _("cms.pages.create") }}    -> "Créer une page" (fr)
  {{ _("cms.publishing.draft") }} -> "Entwurf" (de)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

app/modules/cms/definition.py

@@ -63,6 +63,8 @@ cms_module = ModuleDefinition(
     exceptions_path="app.modules.cms.exceptions",
     # Templates remain in core for now (admin/content-pages*.html)
     templates_path=None,
+    # Module-specific translations (accessible via cms.* keys)
+    locales_path="locales",
 )