feat: MVP-5 Sprint 1 - Backup/Restore system

- Add backup types and RecallBackup format
- Create backup snapshot engine (createBackupSnapshot)
- Add IndexedDB storage for local backups
- Implement retention policy (max 10, 30-day cleanup)
- Add backup validation and restore logic (merge/replace modes)
- Add backup restore UI dialog with preview and confirmation
- Add unsaved changes guard hook
- Integrate backups section in Settings
- Add backup endpoint to export-import API

src/lib/backup-policy.ts

@@ -0,0 +1,56 @@
+import { getBackups } from '@/lib/backup-storage'
+import { deleteBackup } from '@/lib/backup-storage'
+import { BackupSource } from '@/types/backup'
+
+const MAX_AUTOMATIC_BACKUPS = 10
+const MAX_BACKUP_AGE_DAYS = 30
+
+function daysAgo(date: Date): number {
+  const now = new Date()
+  const diffMs = now.getTime() - date.getTime()
+  return diffMs / (1000 * 60 * 60 * 24)
+}
+
+export async function shouldCleanup(): Promise<boolean> {
+  const backups = await getBackups()
+
+  const automaticBackups = backups.filter((b) => b.source === 'automatic')
+  if (automaticBackups.length > MAX_AUTOMATIC_BACKUPS) {
+    return true
+  }
+
+  const oldBackups = backups.filter(
+    (b) => daysAgo(new Date(b.createdAt)) > MAX_BACKUP_AGE_DAYS
+  )
+  if (oldBackups.length > 0) {
+    return true
+  }
+
+  return false
+}
+
+export async function cleanupOldBackups(): Promise<number> {
+  const backups = await getBackups()
+  let deletedCount = 0
+
+  // Remove automatic backups exceeding the limit
+  const automaticBackups = backups.filter((b) => b.source === 'automatic')
+  if (automaticBackups.length > MAX_AUTOMATIC_BACKUPS) {
+    const toRemove = automaticBackups.slice(MAX_AUTOMATIC_BACKUPS)
+    for (const backup of toRemove) {
+      await deleteBackup(backup.id)
+      deletedCount++
+    }
+  }
+
+  // Remove backups older than 30 days
+  const recentBackups = await getBackups()
+  for (const backup of recentBackups) {
+    if (daysAgo(new Date(backup.createdAt)) > MAX_BACKUP_AGE_DAYS) {
+      await deleteBackup(backup.id)
+      deletedCount++
+    }
+  }
+
+  return deletedCount
+}