Documents OXICLOUD_ENABLE_VIDEO_THUMBNAILS (+ OXICLOUD_FFMPEG_PATH) in
example.env and docs/config/env.md — closes the discoverability gap
where the env var was only visible in Rust docstrings.
Also lands docs/plan/bundled-binary.md — the design record referenced
from code comments in src/cli/mod.rs, src/interfaces/web/embedded.rs,
and the Dockerfile.
Edouard Vanbelle