feat(fleet): give the media servers an I/O share that actually exists

A Plex section scan walked 50k files at ~850 r/s and nothing bounded it.
`IOWeight=` in a quadlet looks like the fix and does nothing: systemd delegates
`cpu memory pids` to user@.service and NOT io, and it drops a resource line it
cannot honour without a word — the container boots healthy and its cgroup has no
io.weight file at all. Same family as RequiresMountsFor= (inert in the user
manager) and ConditionPathIsMountPoint= (fail-dead).

Place the two system drop-ins that make the controller reachable (user.slice
IOAccounting=yes puts io in user.slice/cgroup.subtree_control, measured
immediately; user@1000.service DelegateControllers gains io and lands at the
next reboot, because restarting the user manager would kill every rootless
container on the host), then ask for the share: IOWeight=10 on jellyfin and plex.
10 is a convention, not a measured optimum — proportional share, only bites
under contention, only requirement is far below the default 100.

ensure_user_io_delegate places both from git and prints NOTE: io is not
delegated … inert until the next reboot, so the quadlet line never reads as live
config. Verified on the host: subtree_control flipped to `cpu io memory pids`,
the drop-in is attached and parsed (systemctl cat), and the podman 5.7 generator
emits IOWeight=10 for both units (bare key; UnitConfigItem= not needed).

Two limits recorded instead of hidden: a container's weight covers its whole
cgroup (scan and playback share it), and the comparison happens inside
user.slice, so it cannot push a rootless unit below a root unit such as
frank-archon — that pairing is user.slice's own weight, a separate call.

Docs: Quadlet Lifecycle 4b (the three-command read of the chain), Pi-hole (all
rootless containers share one 10.0.2.100, so one chatty container throttles DNS
for the host — 8 events today), E2E Suite gotcha 7 (the gate depends on the
runner's resolver: 21 reds on a lost wired link, 37/37 minutes later).

Seam: an in-repo IOWeight= may only exist when both drop-ins exist AND the
deployer places them. ./tests/run.sh: 789 ok, exit 0.

scripts/deploy-remote.sh

@@ -1208,14 +1208,47 @@ ensure_media_view() {
     # array below stays read-only and every sidecar Jellyfin writes lands on the
     # NVMe.
     # Raise it BEFORE either container starts — a start against a bare view
-    # directory would let a scan clear the library database. The units also carry
-    # ConditionPathIsMountPoint, so a missing view skips the start rather than
-    # damaging anything. A relayout must be followed by `media-view.sh reap`
+    # directory would let a scan clear the library database. Each unit gates on
+    # scripts/wait-for-mount.sh, which waits and then refuses the start. A
+    # relayout must be followed by `media-view.sh reap`
     # (see the Media Relayout page), which this is not.
     run_privileged bash scripts/media-view.sh install
     bash scripts/media-view.sh verify
 }
 
+ensure_user_io_delegate() {
+    # The media servers ask for an I/O share (IOWeight= in their quadlets) so a
+    # library scan cannot starve playback or the array's other reader. Two system
+    # drop-ins are what make that request real, and both are host state, so the
+    # deployer places them:
+    #   user.slice.d/io-accounting.conf        io enters user.slice's subtree
+    #   user@1000.service.d/io-delegate.conf   the user manager gets the controller
+    # Without the first, `io` never appears in user.slice/cgroup.subtree_control,
+    # so no container under it can have an io.weight file at all; without the
+    # second the user manager never asks for the controller. systemd hands
+    # user@.service cpu/memory/pids by default and drops a line it cannot honour
+    # without a word — IOWeight= on its own is inert config.
+    local src dst
+    for src in \
+        dankserver/host/systemd/user.slice.d/io-accounting.conf \
+        dankserver/host/systemd/user@1000.service.d/io-delegate.conf; do
+        dst=/etc/systemd/system/${src#dankserver/host/systemd/}
+        run_privileged mkdir -p "$(dirname "$dst")"
+        if ! run_privileged cmp -s "$src" "$dst"; then
+            run_privileged install -m 644 "$src" "$dst"
+            run_privileged systemctl daemon-reload
+        fi
+    done
+    # Accounting lands immediately; the delegation is read when user@1000.service
+    # starts, and restarting that unit would kill every rootless container on this
+    # host. So the share becomes real at the next reboot — say so out loud rather
+    # than let a quadlet line read as live config.
+    if ! systemctl show user@1000.service -p DelegateControllers | grep -qw io; then
+        echo "NOTE: io is not delegated to user@1000.service yet - IOWeight= is"
+        echo "NOTE: inert until the next reboot (see [[Quadlet Lifecycle]])"
+    fi
+}
+
 ensure_provider_keys() {
     # Jellyfin's metadata-provider keys live in the container-owned plugin
     # configuration under the data root, never in git; scripts/jellyfin-providers.sh
@@ -1237,6 +1270,7 @@ ensure_provider_keys() {
 
 deploy_jellyfin() {
     ensure_media_view
+    ensure_user_io_delegate
     ensure_provider_keys
     cp "dankserver/jellyfin/quadlet/jellyfin.container" "$quadlet_unit_dir/"
     systemctl --user daemon-reload
@@ -1326,6 +1360,7 @@ deploy_copyparty() {
 
 deploy_plex() {
     ensure_media_view
+    ensure_user_io_delegate
     cp "dankserver/plex/quadlet/plex.container" "$quadlet_unit_dir/"
     systemctl --user daemon-reload
     if ! systemctl --user is-active -q plex.service; then