fix(fleet): refuse a container start until the mount it binds has answered

Dankserver rebooted with the array's mount and the media containers racing. A
rootless quadlet cannot order itself against the system's mnt-entropy.mount, so
`RequiresMountsFor=/mnt/entropy` is inert in the user manager: podman resolved
the bind against the pre-mount directory, Jellyfin and Plex came up blind, and
Jellyfin read an empty library and began `Removing item, Type: "Season"` by the
thousand. Copyparty had no guard at all.

`scripts/wait-for-mount.sh` is the gate: it waits for every named mountpoint,
then refuses by name. `ConditionPathIsMountPoint=` is NOT the fix — measured on
the host, an unmet condition logs `skipped, unmet condition check` with
NRestarts=0 and no start attempt ever follows, which is fail-dead, not
fail-closed. Jellyfin and Plex swap the condition for the gate; Copyparty gets
one; the law is recorded in AGENTS.md and the view page.

Verified on the host: the 5.7.0 quadlet generator accepts ExecStartPre in
[Service] (exit 0, all three units generated, gate present), the gate waits for
a mount that appears mid-window and proceeds, and it refuses a plain directory
after its window naming the path.

tests/media_overlay.bats pinned the old mechanism by name; its intent — both
media servers refuse to start without the view — now rides the gate.

dankserver/jellyfin/quadlet/jellyfin.container

@@ -43,11 +43,10 @@ PublishPort=1900:1900/udp
 Volume=/home/dank/Containers/jellyfin/config:/config
 Volume=/home/dank/Containers/jellyfin/cache:/cache
 # The view, writable: Jellyfin is the metadata writer (movie.nfo / tvshow.nfo /
-# episode.nfo + artwork) and those bytes belong on the SSD. The condition is the
-# fail-closed guard — starting against a plain, unmounted view directory would let
-# a scan conclude the library is empty.
+# episode.nfo + artwork) and those bytes belong on the SSD. The gate in [Service]
+# is the guard: started against a plain, unmounted view directory a scan decides
+# the library is empty and deletes what it cannot see.
 Volume=/var/lib/media-meta/view:/data
-UnitConfigItem=ConditionPathIsMountPoint=/var/lib/media-meta/view
 AddDevice=/dev/dri
 GroupAdd=keep-groups
 HealthCmd=curl -f http://localhost:8096/health
@@ -56,6 +55,9 @@ HealthTimeout=10s
 HealthRetries=3
 
 [Service]
+# Refuse to start until the bind source is a real mount — see the header of
+# scripts/wait-for-mount.sh for the blind-boot incident this prevents.
+ExecStartPre=/home/dank/serverAppConfigs/scripts/wait-for-mount.sh /var/lib/media-meta/view /mnt/entropy
 Restart=always
 RestartSec=5