feat(ai/stt): add dictation language selection to STT settings

Add a per-workspace `sttLanguage` setting (ISO-639-1 hint; empty =
auto-detect) and a searchable language picker in the Voice / STT settings
card. The hint is forwarded to the transcription endpoint:
- multipart path via the AI SDK `providerOptions.openai.language`
- JSON (OpenRouter) path via a top-level `language` body field
only when non-empty, so auto-detect behaves exactly as before.

Threaded through the whole stack: ai.types, update DTO, AiSettingsService
(resolve/getMasked/update), the workspace.repo SQL allowlist, the client
ai-settings service types, and the provider-settings form. Adds en-US
source keys and ru-RU translations.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -1123,5 +1123,8 @@
   "Added {{name}} to favorites": "{{name}} добавлено в избранное",
   "Removed {{name}} from favorites": "{{name}} удалено из избранного",
   "Page menu for {{name}}": "Меню страницы для {{name}}",
-  "Create subpage of {{name}}": "Создать подстраницу для {{name}}"
+  "Create subpage of {{name}}": "Создать подстраницу для {{name}}",
+  "Dictation language": "Язык диктовки",
+  "Auto-detect": "Автоопределение",
+  "Spoken language hint sent to the transcription model. Auto-detect lets the model decide.": "Подсказка языка речи для модели транскрипции. «Автоопределение» оставляет выбор за моделью."
 }