feat(backup): carry per-schedule units-per-dose through backup and restore

The schedule exporter enumerates columns, so the new per-schedule
units-per-dose needs adding by hand: export it as a Decimal string in
the disaster-recovery payload, accept it in the wire schema, and write
it back on restore (NULL stays NULL to inherit). Extend the round-trip
test to prove a half-tablet slot survives export and re-import.

src/lib/validations/backup.ts

@@ -127,6 +127,9 @@ const medicationScheduleSchema = z
     windowEnd: z.string().min(1),
     label: z.string().nullable().optional(),
     dose: z.string().nullable().optional(),
+    // #219 — per-schedule units per dose. Serialised as a Decimal string in a
+    // DR file (or a number in a hand-authored one); NULL means inherit.
+    unitsPerDose: z.union([z.string(), z.number()]).nullable().optional(),
     daysOfWeek: z.string().nullable().optional(),
     timesOfDay: z.array(z.string()).optional(),
     reminderGraceMinutes: z.number().int().nullable().optional(),