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/features/admin/mobile/AdminBackupMobileScreen.tsx

@@ -773,7 +773,7 @@ export const AdminBackupMobileScreen: React.FC = () => {
                         </span>
                       </div>
 
-                      <div className="flex items-center gap-4 mb-3 text-xs text-slate-600">
+                      <div className="flex flex-wrap items-center gap-2 mb-3 text-xs text-slate-600">
                         <span
                           className={`px-2 py-1 rounded ${
                             backup.backupType === 'scheduled'
@@ -784,6 +784,11 @@ export const AdminBackupMobileScreen: React.FC = () => {
                           {backup.backupType}
                         </span>
                         <span>{formatFileSize(backup.fileSizeBytes)}</span>
+                        {backup.expiresAt && (
+                          <span className="text-slate-500">
+                            Expires: {formatDate(backup.expiresAt)}
+                          </span>
+                        )}
                       </div>
 
                       <div className="flex gap-2">

frontend/src/features/admin/types/admin.types.ts

@@ -288,6 +288,7 @@ export interface PromoteToAdminRequest {
 export type BackupFrequency = 'hourly' | 'daily' | 'weekly' | 'monthly';
 export type BackupType = 'scheduled' | 'manual';
 export type BackupStatus = 'in_progress' | 'completed' | 'failed';
+export type BackupCategory = 'hourly' | 'daily' | 'weekly' | 'monthly';
 
 export interface BackupHistory {
   id: string;
@@ -304,6 +305,8 @@ export interface BackupHistory {
   completedAt: string | null;
   createdBy: string | null;
   metadata: Record<string, unknown>;
+  categories: BackupCategory[];
+  expiresAt: string | null;
 }
 
 export interface BackupSchedule {

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"