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.

.env.example

@@ -54,3 +54,11 @@ SMTP_FROM=
 # more than one instance against the same database, keep SCHEDULER_ENABLED=true on exactly one
 # of them and false on the rest, to avoid double quota use and write races.
 SCHEDULER_ENABLED=true
+
+# --- Download center ---
+# The Download Center adds a `worker` container (runs the yt-dlp/ffmpeg job loop) and a small
+# `bgutil-pot` sidecar (mints YouTube tokens) — both come up automatically with docker compose.
+# Downloaded media defaults to a Docker-managed named volume. Set DOWNLOAD_HOST_PATH to a host
+# directory instead — e.g. one your Plex server can read — to keep the Plex-style tree there.
+# The path must be writable by the container user (uid of `appuser`, 1000): chown 1000:1000 <dir>.
+# DOWNLOAD_HOST_PATH=/mnt/media/youtube