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/app/api/export-import/route.ts

@@ -3,9 +3,18 @@ import { prisma } from '@/lib/prisma'
 import { noteSchema, NoteInput } from '@/lib/validators'
 import { createErrorResponse, createSuccessResponse, ValidationError } from '@/lib/errors'
 import { syncBacklinks } from '@/lib/backlinks'
+import { createBackupSnapshot } from '@/lib/backup'
 
-export async function GET() {
+export async function GET(req: NextRequest) {
   try {
+    const { searchParams } = new URL(req.url)
+    const format = searchParams.get('format')
+
+    if (format === 'backup') {
+      const backup = await createBackupSnapshot('manual')
+      return createSuccessResponse(backup)
+    }
+
     const notes = await prisma.note.findMany({
       include: { tags: { include: { tag: true } } },
     })