refactor(deploy): share UP Board serial hardware between installed + live ISO

The sintra live ISO (live.nix) had no serial support — ftdi_sio and the
ttyJ5/ttyJ7 udev symlinks were only in hardware/upboard.nix (installed), so
booting iso-sintra on real hardware failed on the validator + F56 dispenser
while the disk image worked. The two definitions had already drifted (live's
tejo block lacked ttyS4).

Extract the UP Board serial peripherals (usbserial/ftdi_sio/cp210x, the
ttyJ4/ttyJ5/ttyJ7 udev symlinks + permissions, console=tty0) into
hardware/upboard-serial.nix and import it from both upboard.nix (installed
tejo + sintra) and live.nix (sintra only). Single source of truth — the two
artifacts can't drift again. Named upboard-serial (not sintra-serial) since
upboard.nix serves both tejo-installed and sintra-installed.

Camera + LED/SPI rules stay inline in upboard.nix (installed-specific; the
pairing camera works via getUserMedia without the scanner symlink). Verified
by eval: live sintra now carries ftdi_sio + console=tty0 + ttyJ7; installed
sintra/tejo unchanged (serial present, camera present, no console dupe).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

deploy/nixos/live.nix

@@ -48,7 +48,12 @@ in
 
     # Reuse ATM systemd service module
     ./bitspire-atm.nix
-  ];
+  ]
+  # Sintra: share the UP Board serial hardware (validator/dispenser/printer
+  # modules + udev symlinks + console=tty0) with the installed image so the
+  # live ISO drives the same hardware. Safe to import here — unlike upboard.nix
+  # it declares no fileSystems, so there's no live-boot mount conflict.
+  ++ lib.optionals (machineModel == "sintra") [ ./hardware/upboard-serial.nix ];
 
   # ISO image settings
   image.fileName = "bitspire-${machineModel}-live.iso";