feat(setup): USB printer discovery — pick a real /dev/usb device

The kernel numbers usblp nodes by plug/boot order (park-buzi's printer
is lp1); the wizard hardcoded lp0 in labels/default and the admin had to
shell in and `ls /dev/usb`. Now:

- GET /api/setup/usb-printers enumerates /dev/usb/lpN (visible via the
  compose bind-mount) and enriches each with the printer's self-reported
  make/model from sysfs ieee1284_id (readable through Docker's ro /sys).
- The wizard's devicePath becomes a SELECT of printers actually present
  ("/dev/usb/lp1 — Xprinter XP-K200L"): a fresh form preselects the
  first real device; a saved-but-unplugged path stays selectable,
  flagged "saved — not present now"; zero found falls back to free text
  + a check-the-cable hint.
- Transport option label no longer hardcodes lp0.

Wiki: printer-usb-transport marked HARDWARE-VERIFIED (lab 2026-07-07:
full slip + feed + cut over USB — parity with TCP).

Claude-Session: https://claude.ai/code/session_01Xcm6ikLgGoCxxHrxtjkk5V

wiki/concepts/printer-usb-transport.md

@@ -121,6 +121,19 @@ completion signal, but Node cannot poll an arbitrary char-device fd without a na
 hold-back + drain gets the same guarantee for all but the final byte, whose packet the printer
 ACKs immediately after having just freed its buffer.)
 
+**✅ HARDWARE-VERIFIED (lab bench, 2026-07-07):** with both fixes, the ICS XP-K200L over USB
+prints the complete slip, feeds, and CUTS — parity with TCP. The USB transport is done.
+
+**Device discovery (2026-07-07).** The kernel numbers usblp nodes by plug/boot order — park-buzi's
+printer is `lp1`, and the admin had to shell in and `ls /dev/usb` to learn that. The wizard now
+lists REAL printers: `GET /api/setup/usb-printers` enumerates `/dev/usb/lpN` (visible via the
+compose bind-mount) and enriches each with the printer's self-reported make/model from sysfs
+(`/sys/class/usbmisc/lpN/device/ieee1284_id` — readable through Docker's default ro `/sys`). The
+devicePath field becomes a SELECT ("/dev/usb/lp1 — Xprinter XP-K200L") with a fresh form
+preselecting the first present device; a saved-but-unplugged path stays selectable, flagged
+"saved — not present now"; zero devices found falls back to the free-text path + a check-the-cable
+hint. The transport option label no longer hardcodes lp0.
+
 > Driver-choice note for this clone: the ICS XP-K200L does NOT serve the Rongta `/prn_stat.htm`
 > status page (checked on hardware at 10.0.10.11 — print socket 9100 open, status page absent),
 > so on NETWORK the honest driver is **cashino** (reachability-only monitoring); under `rongta`