fix: Implement tiered backup retention classification (refs #6)

Replace per-schedule count-based retention with unified tiered classification.
Backups are now classified by timestamp into categories (hourly/daily/weekly/monthly)
and are only deleted when they exceed ALL applicable category quotas.

Changes:
- Add backup-classification.service.ts for timestamp-based classification
- Rewrite backup-retention.service.ts with tiered logic
- Add categories and expires_at columns to backup_history
- Add Expires column to desktop and mobile backup UI
- Add unit tests for classification logic (22 tests)

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

frontend/src/pages/admin/AdminBackupPage.tsx

@@ -386,6 +386,7 @@ export const AdminBackupPage: React.FC = () => {
                   <TableCell>Size</TableCell>
                   <TableCell>Status</TableCell>
                   <TableCell>Created</TableCell>
+                  <TableCell>Expires</TableCell>
                   <TableCell align="right">Actions</TableCell>
                 </TableRow>
               </TableHead>
@@ -415,6 +416,9 @@ export const AdminBackupPage: React.FC = () => {
                       />
                     </TableCell>
                     <TableCell>{formatDate(backup.startedAt)}</TableCell>
+                    <TableCell>
+                      {backup.expiresAt ? formatDate(backup.expiresAt) : '-'}
+                    </TableCell>
                     <TableCell align="right">
                       <IconButton
                         size="small"