v0.3.1 — PO Token plugin + fresh-install fix

This release fixes a critical bug that made the app unusable on a fresh install, and completes the one-and-done PO Token setup story.
Bug fixes

Fixed a fresh-install regression where every yt-dlp call (indexing, downloading, version checks) would abort with Invalid plugin directory: /config/yt-dlp-plugins if the plugin directory didn't exist. The app now fails open — if the plugin directory is absent or empty, yt-dlp runs normally without the flag.

Improvements

The bgutil POT provider yt-dlp plugin is now baked into the image. Adding the bgutil sidecar container to your compose is all that's required for PO tokens to work — no manual plugin download or volume mount needed.
Added download_prevented_reason field on media items, surfaced on the media item page. Distinguishes between manually prevented, policy-blocked, and error-stopped downloads.
Added docker-compose.sample.yml to the repo — a complete annotated three-container compose file for new installs.

Notes

The baked-in plugin is pinned to bgutil 1.3.1. Run the matching sidecar tag (brainicism/bgutil-ytdlp-pot-provider:1.3.1).
If you were previously running :dev, this release is what :latest now points to.