Introduce a `/tracks/:id/flac` endpoint and `open_cmaf_stream` client method to enable incremental decryption and caching of FLAC audio. Refactor segment decryption into a reusable helper and stream decrypted frames via a bounded tokio channel. Add `tokio-util` as an optional dependency, update the `pmoserver` feature, and implement conditional routing for local proxy versus direct API access. Update the architecture roadmap to document these improvements.
