feat: add configurable backup time for local and USB backups

Replace hardcoded 8 PM / 9 PM backup schedules with user-selectable
hour dropdowns. Adds autoBackupHour and usbBackupHour fields to User
model. Cron jobs now check every hour against the configured time.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

packages/db/prisma/schema.prisma

@@ -23,7 +23,9 @@ model User {
   username             String                @unique
   password             String
   autoBackupEnabled    Boolean               @default(true)
+  autoBackupHour       Int                   @default(20)
   usbBackupEnabled     Boolean               @default(false)
+  usbBackupHour        Int                   @default(21)
   patients             Patient[]
   appointments         Appointment[]
   staff                Staff[]