feat(audiobookshelf): the audiobook server as a boot-persistent quadlet

Audiobookshelf serves /mnt/entropy/Audiobooks to browsers and phones with its own
library and per-listener positions. The design point of the unit: every byte this
server writes is its own state - SQLite library, accounts, progress, cached covers
- and all of it lands on the NVMe under /home/dank/Containers/audiobookshelf. The
audio bind is :ro, so a stray sidecar write onto the spindle is structurally
impossible rather than merely discouraged, and the array keeps exactly one
sanctioned write path (scripts/media-view.sh).

Three failure modes the unit is shaped around:

- Bind-source ordering. In a rootless unit RequiresMountsFor= is inert and
  ConditionPathIsMountPoint= is fail-dead, so a missing array does not stop the
  start: podman invents an empty directory and the first scan records an empty
  library as truth. ExecStartPre=wait-for-mount.sh is the gate that works here,
  the same one copyparty uses.
- The health binary. The image is Alpine: wget, no curl. A curl HealthCmd reads a
  healthy server as unhealthy and Restart=always turns that into a loop.
- The image pin. podman records the linux/amd64 manifest for what it pulled, not
  the manifest-list digest `latest` points at, so the amd64 digest is the pin that
  names the build this host runs. Upstream version tags stop at 2.19.0 while
  `latest` is newer, so latest@digest is the only form that is both current and
  exact - the jellyfin/romm precedent.

The first-boot scan reads the whole tree once, so IOWeight=10 keeps it behind
playback; that weight is only honoured because the user.slice accounting drop-in
and the user@1000 delegation are installed by the deployer.

Deployer registration is the standard four touch-points (client gate, remote
allowlist + handler + dispatcher, planner order/set/path map, test routing). The
handler creates both state roots before the first start - letting podman invent
them leaves the SQLite file unwritable by the container user under a rootless
first boot - and gates on /healthcheck.

scripts/test-changed.sh

@@ -334,7 +334,7 @@ for path in "${CHANGED[@]}"; do
         # Cutover-era dankserver quadlet units (romm trio, jellyfin, calibre-web,
         # copyparty, plex, tautulli): the seam checks boot-persistence +
         # v1-generator contract; deploy-flow seams guard the deployer routing.
-        dankserver/romm/*|dankserver/jellyfin/*|dankserver/calibre-web/*|dankserver/copyparty/*|dankserver/plex/*|dankserver/tautulli/*)
+        dankserver/romm/*|dankserver/jellyfin/*|dankserver/calibre-web/*|dankserver/copyparty/*|dankserver/plex/*|dankserver/tautulli/*|dankserver/audiobookshelf/*)
             add_test tests/dankserver_quadlet.bats
             add_test tests/deployment_flow.bats
             add_test tests/deployment_scoping.bats