Phases 0-1 of #701. Drops dead deps, moves vosk to an optional extra, adds uv.lock and hash-pinned requirements exports, pins pywhispercpp to 1.5.0.
Closes#705. Refs #701.
Stacked bottom of #701. Replaces Makefile with justfile and hardens install.sh (strict mode, logging, HTTP probes, curl -f on model downloads, no v0.10.1-beta fallback).
Closes#700.