fix: resolve all 23 MAINTENANCE audit tickets

- deps guards before -h|--help in docker-health/ps, network-scan,
  usb-server, media-mp3/mp4 (--dry-run pre-scan kept); system-firewall
  gains usage()/--help; autostart/usb-automount get flags.sh + template
- install.sh: normalize N-M range syntax in --steps
- bin/pos: INTERACTIVE_CMDS += docker-compose docker-vbox network-hotspot
- common.sh: canonical XDG-aware CONFIG_DIR + DIM color var; notify.sh
  stderr fallback; ent_plugin_* registry renames (runtime plugin API kept)
- docker-compose SCALE_DIR/CONFIG_ENV env seams; ffmpeg in PACKAGES;
  scrcpy.sh exec bit
- docs: health is console-only (--send/--markdown removed), POS.md file
  refs for config/tree/entertainment, DEV.md no-guard exception, docmap/
  filetable regenerated (make gen), hand-maintained line rows bumped
- add scripts/lint-conventions.sh gate + Makefile lint target; record
  all VERIFIED outcomes in MAINTENANCE.md; AGENT_TODO Done entry
  (2026-08-14)
- gates: make gen/check/lint all green (0 FAIL, 0 WARN); bash -n sweep
  clean; restricted-PATH dep tests + step-matrix dry-runs verified

preinstall.sh

@@ -37,6 +37,7 @@ PACKAGES=(
     lm-sensors smartmontools nvme-cli hdparm
     sysstat iotop atop vnstat
     python3 python3-pip rclone
+    ffmpeg
     libqrencode4 libgtk-3-0 adb
 )