feat: single daemon with local OpenAI-compatible STT service

[krystophny]

Summary

• add in-process HTTP service with OpenAI-compatible endpoints (/v1/audio/transcriptions, /v1/audio/translations, /healthz)
• run service alongside the daemon hotkey loop in the same process
• add service config + CLI/env overrides for bind host/port, request timeout, upload limits, and allowed languages
• add request-level language/prompt overrides in whisper transcriber via transcribe_with_options
• add end-to-end tests that use existing RemoteTranscriber client against the new local service

Design notes

• no auth in voxtype service (loopback-first)
• service defaults to constrained language set (de, en) when request language is not pinned
• WAV decode/downmix/resample to 16k mono is handled server-side

Testing

• cargo test

Closes #244