Implement bulkgen/providers/bfl.py with a fully async httpx-based client
that supports all current and future BFL models (including flux-2-*).
Remove the blackforest dependency and simplify the image provider by
eliminating the asyncio.to_thread wrapper.
- Replace pyright with basedpyright in devenv.nix (custom hook)
- Add basedpyright to devenv packages
- Fix all basedpyright warnings: add DiGraph[str] type args, annotate
class attributes, narrow SyncResponse, handle unused call results,
suppress unavoidable Any from yaml.safe_load and untyped blackforest
- Replace pydantic-settings[yaml] with direct pyyaml dependency
- Update CLAUDE.md to reflect basedpyright and dependency changes
