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.

install.ps1

@@ -28,6 +28,10 @@ POSTGRES_PASSWORD=$pgPass
 SECRET_KEY=$secretKey
 TOKEN_ENCRYPTION_KEY=$fernet
 OAUTH_REDIRECT_URL=$url/auth/callback
+
+# Optional: store Download Center media in a host folder (e.g. one your Plex server reads) instead
+# of a Docker volume. Must be writable by uid 1000 (chown -R 1000:1000 <dir>).
+# DOWNLOAD_HOST_PATH=/mnt/media/youtube
 "@ | Set-Content -Path .env -Encoding ascii
   Write-Host "Wrote .env (secrets generated)."
 }