EU data residency for transcription: configurable AssemblyAI region + OpenAI-compatible provider (Voxtral)

[Rachdevop]

Problem

Self-hosted Cap users in the EU cannot keep transcription data in the EU.

Cap's transcription is hardcoded to AssemblyAI's US endpoint (api.assemblyai.com) in workflows/transcribe.ts. AssemblyAI does offer an EU endpoint (api.eu.assemblyai.com, Dublin) that uses the same API key and is available on the free tier, but Cap provides no way to select it.

Requests

1. Configurable AssemblyAI region / base URL
   Add an env var (e.g. ASSEMBLYAI_BASE_URL or ASSEMBLYAI_REGION=eu) so EU self-hosters can point transcription at api.eu.assemblyai.com.

2. OpenAI-compatible transcription provider (e.g. Mistral Voxtral)
   Support any /v1/audio/transcriptions endpoint as a transcription provider. This would let EU users use Mistral's Voxtral (100% EU, ZDR) instead of AssemblyAI US.

Context

• Complementary to Support custom base URLs for self-hosted AI providers (vLLM, self-hosted Whisper, etc.) #1996 (base-URL overrides), but focused on transcription data residency / GDPR rather than self-hosted Whisper.
• For AI text (title/summary/chapters), EU users already have a workaround via AI_PROVIDER=openai-compatible + Mistral. There is no equivalent for transcription.

[linear]

CAP-797