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.
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