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.

tests/media_overlay.bats

@@ -61,7 +61,10 @@ UNIT="$REPO_ROOT/dankserver/host/systemd/media-meta-view.mount"
 @test "both media servers bind the view and refuse to start without it" {
     for unit in "$JELLY" "$PLEX"; do
         grep -q "^Volume=$MOUNT:" "$unit" || return 1
-        grep -q "^UnitConfigItem=ConditionPathIsMountPoint=$MOUNT$" "$unit" || return 1
+        # The refusal is a start GATE, not a Condition: measured on the host, a
+        # unit whose Condition goes unmet is skipped forever (NRestarts=0, no
+        # start attempt) — fail-dead. The gate waits, then refuses.
+        grep -q "^ExecStartPre=.*wait-for-mount.sh .*$MOUNT" "$unit" || return 1
         # The raw array must no longer be a container source: that bind is what
         # made sidecar sharing impossible (read-only) and metadata slow (seeks).
         ! grep -q '^Volume=/mnt/entropy' "$unit"