feat(media): share one metadata set between Jellyfin and Plex over an SSD overlay

Both media servers read a Kodi-format sidecar (movie.nfo / tvshow.nfo /
episode.nfo) only when it sits next to the media file, and the library lives on
the RAID6 array, deliberately mounted read-only for every container and slow to
seek for a thousand tiny XML reads. So neither server binds the array any more:
both bind /mnt/media, an OverlayFS merge of the array (lowerdir, never written)
with /var/lib/media-meta on the NVMe (upperdir, where every write lands).

Jellyfin binds the view rw at /data and is the fleet's only metadata writer;
Plex binds it ro at /mnt/entropy, the container path its own database already
recorded, so only the host side of that bind moved and its library paths keep
resolving.

scripts/media-view.sh plans, installs, verifies, reports and reaps the view. It
refuses to mount over an unmounted array, refuses an upper layer on rotational
storage, and refuses a non-root install. deploy-remote.sh raises the view before
it restarts either media server, both units carry
ConditionPathIsMountPoint=/mnt/media so a missing view skips the start instead of
letting a scan clear the library database, and the planner routes the installer
and dankserver/host/** to the jellyfin+plex pair.

tests/media_overlay.bats (11 seams) pins the resolved shape, both refusals, the
two binds and their guard, the deployer's ordering, the planner mapping, and the
reap behaviour against a fixture (ghosts listed, then moved to the SSD trash --
files that exist only above are sidecars, not ghosts, so only directories are
candidates).

dankserver/host/systemd/mnt-media.mount

@@ -0,0 +1,28 @@
+# /etc/systemd/system/mnt-media.mount
+#
+# Installed by `scripts/media-view.sh install` — the repo copy is the source of
+# truth (fleet law: never edit host state by hand).
+#
+# An OverlayFS merge that shows the media library as one tree while splitting
+# where its bytes live: lowerdir is the RAID6 array (the source of truth, never
+# written), upperdir is a directory on the NVMe that receives every write. The
+# two media servers bind the result at /mnt/media, so Jellyfin's Kodi-format
+# sidecars and artwork are SSD bytes — which is what makes them fast to browse
+# and what keeps Plex's NFO agent off the spinners — while the media itself stays
+# on the array untouched.
+#
+# RequiresMountsFor is load-bearing: without the array mounted, lowerdir is an
+# empty directory and the view would present an empty library as truth.
+
+[Unit]
+Description=SSD-backed overlay view of the media library (array + NVMe metadata layer)
+RequiresMountsFor=/mnt/entropy
+
+[Mount]
+What=overlay
+Where=/mnt/media
+Type=overlay
+Options=lowerdir=/mnt/entropy,upperdir=/var/lib/media-meta/upper,workdir=/var/lib/media-meta/work
+
+[Install]
+WantedBy=local-fs.target