feat(backup): replace JSON import/export with SQLite database backup system

Replace the JSON-based import/export with a proper backup system that copies
the SQLite database file directly. Supports manual on-demand backups, periodic
scheduled backups via node-cron, configurable retention, file download, and
full database restore.

- Add backupService with VACUUM INTO for safe DB copies
- Add backupScheduler following healthcheckScheduler pattern
- Add 6 admin API endpoints (create, list, download, restore, delete, schedule)
- Add BackupPanel UI with backup table, confirmation dialogs, schedule config
- Add backup fields to SystemSettings schema
- Remove old ImportExportPanel, exportService, importService, and related code

prisma/schema.prisma

@@ -201,6 +201,9 @@ model SystemSettings {
   healthcheckDefaults   String   @default("{}") // JSON stored as string for SQLite
   customCss             String?
   onboardingComplete    Boolean  @default(false)
+  backupEnabled         Boolean  @default(false)
+  backupCronExpression  String   @default("0 3 * * *") // default: daily at 3 AM
+  backupMaxCount        Int      @default(10)
   createdAt             DateTime @default(now())
   updatedAt             DateTime @updatedAt
 }