feat(ai-chat): show RAG indexing coverage in AI settings

Display "Indexed N of M pages" on the AI provider settings page so admins
can see how much of the wiki is covered by vector-RAG semantic search.

- page-embedding.repo: add countIndexedPages() — distinct non-deleted pages
  that have stored embeddings in the workspace
- page.repo: add countByWorkspace() — total non-deleted pages
- ai-settings.service: compute both counts in getMasked() (Promise.all) and
  return them with the masked settings; inject PageEmbeddingRepo + PageRepo
- MaskedAiSettings / IAiSettings: add indexedPages + totalPages
- ai-provider-settings: render a dimmed coverage line under "Embedding model"
- i18n: add the "Indexed {{indexed}} of {{total}} pages" key (en-US, ru-RU)

apps/client/public/locales/ru-RU/translation.json

@@ -92,6 +92,7 @@
   "Import pages": "Импорт страниц",
   "Import pages & space settings": "Импорт страниц и настройки пространства",
   "Importing pages": "Импортирование страниц",
+  "Indexed {{indexed}} of {{total}} pages": "Проиндексировано {{indexed}} из {{total}} страниц",
   "invalid invitation link": "недействительная ссылка-приглашение",
   "Invitation signup": "Регистрация по приглашению",
   "Invite by email": "Пригласить по электронной почте",