pcb: use the engraver capacitor bank as backup power for the 5V rail

A USB PD power supply may decide to hard reset or otherwise power cycle
the board, assuming we are a battery powered device. We're not, but the
next best thing is the massive capacitor bank that keeps the engraver
well-fed during its activation pulses.

Perform a cheap power OR with Schottky diodes; the voltage drop is ok,
even from a 5V non-PD supply because what really matters is keeping the
3.3V MCU alive.

pcb/fixups_jlcpcb.go

@@ -68,4 +68,6 @@ var JLCPCBFixups = map[string]Fixup{
 	"C478465":     {90, 0, 0},
 	"C2864391":    {90, 0, 0},
 	"C28646265":   {180, 0, 0},
+	"C39197":      {180, 1.356 - 1.95, 0},
+	"C128411":     {180, 0, 0},
 }