refactor(ai): explicit STT request format instead of OpenRouter host-sniffing

Replace the implicit `hostname endsWith openrouter.ai` detection with an
explicit, admin-chosen provider field `sttApiStyle` ('multipart' = OpenAI-
compatible multipart /audio/transcriptions; 'json' = OpenRouter-style JSON +
base64 input_audio). The transcription path now branches on the stored field,
not on the URL — nothing hidden from the admin.

- ai.types: add SttApiStyle + STT_API_STYLES; field on AiProviderSettings and
  MaskedAiSettings (resolved via ResolvedAiConfig).
- update-ai-settings.dto: validate sttApiStyle with @IsIn(STT_API_STYLES).
- ai-settings.service: plumb sttApiStyle through resolve()/getMasked() and the
  non-secret update whitelist; workspace.repo: add it to the ALLOWED array so it
  persists.
- ai.service: drop isOpenRouter(); transcribe() branches on cfg.sttApiStyle;
  rename helper to transcribeJsonBase64 with provider-neutral error text and a
  BadRequestException (400) when the base URL is missing for the JSON style.
- client: SttApiStyle type on IAiSettings/IAiSettingsUpdate; "Request format"
  Select on the Voice/STT settings card; i18n.

apps/server/src/database/repos/workspace/workspace.repo.ts

@@ -239,7 +239,7 @@ export class WorkspaceRepo {
     // is a real jsonb object, never a double-encoded string. The CASE self-heals
     // workspaces whose settings.ai.provider was previously corrupted into an
     // array/string.
-    const ALLOWED = ['driver', 'chatModel', 'embeddingModel', 'baseUrl', 'embeddingBaseUrl', 'sttModel', 'sttBaseUrl', 'systemPrompt'];
+    const ALLOWED = ['driver', 'chatModel', 'embeddingModel', 'baseUrl', 'embeddingBaseUrl', 'sttModel', 'sttBaseUrl', 'sttApiStyle', 'systemPrompt'];
     const entries = Object.entries(provider).filter(
       ([k, v]) => v !== undefined && ALLOWED.includes(k),
     );