chore(release): v0.22.0 — Download Center worker/sidecar in all composes + docs

Prep the Download Center epic (Phase 1 + editor + share) for prod/self-host:
- Dockerfile: create /downloads owned by appuser so a named-volume mount is writable (prod Linux).
- docker-compose.{home,selfhost,yml}: add the 'worker' (yt-dlp/ffmpeg job loop) + 'bgutil-pot'
  (PO-token) services + a downloads mount (DOWNLOAD_ROOT, WORKER_ENABLED). Media defaults to a
  named volume; DOWNLOAD_HOST_PATH points it at a host dir (e.g. a Plex-readable folder).
- README / docs/self-hosting.md / .env.example / install.{sh,ps1}: document the Download Center,
  the two extra containers, and DOWNLOAD_HOST_PATH.
- VERSION 0.22.0 + releaseNotes entry.

README.md

@@ -22,6 +22,9 @@ blocker and SponsorBlock keep working.
   tags to slice the feed by.
 - **Playlists with two-way YouTube sync** — build them locally, keep them in sync in both directions.
 - **In-app player** with resume, plus keyboard/scroll controls.
+- **Download Center** — save videos to the server with yt-dlp in a Plex-friendly layout (format
+  presets, per-user storage quota), trim / crop / split & join them in a built-in editor, then save
+  to your device, share with another user, or hand out a public watch link.
 - **Multi-user** with per-user private state, a shared catalog, and a fair daily API-quota guard.
 - **Self-hosted & private**, with a first-run web setup wizard and the interface in **English,
   Hungarian and German**.