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/deploy.sh

@@ -148,6 +148,7 @@ valid_service() {
         dankclaw:frank-dragoon|dankclaw:frank-zealot|\
         dankserver:romm-mariadb|dankserver:romm-redis|dankserver:romm|\
         dankserver:copyparty|dankserver:jellyfin|dankserver:calibre-web|dankserver:calibre-server|dankserver:plex|dankserver:tautulli|\
+        dankserver:audiobookshelf|\
         dankserver:qbittorrent|dankserver:radarr|dankserver:sonarr|\
         dankserver:lidarr|dankserver:readarr|dankserver:node-exporter|dankserver:cadvisor|dankserver:gpu-exporter|\
         dankserver:prowlarr|dankserver:frank-archon|\