feat: add SHA-256 checksum verification for backup integrity

Addresses community feedback about 0KB corrupted backup files going
undetected after upload.

Implementation:
- Compute SHA-256 hash of final artifact (after compress/encrypt) before upload
- After each storage target upload, download the file back and verify
  the hash matches the local checksum
- If verification fails: mark that target as failed, auto-delete the
  corrupted remote file, and log detailed mismatch info
- Store checksum in BackupRecord model (new `checksum` column)
- Display truncated SHA-256 with copy button in backup records UI

Verification flow per storage target:
  local SHA-256 → upload → download → remote SHA-256 → compare
  - match: mark success
  - mismatch: mark failed + delete corrupted remote file

server/internal/service/backup_record_service.go

@@ -30,6 +30,7 @@ type BackupRecordSummary struct {
 	Status            string     `json:"status"`
 	FileName          string     `json:"fileName"`
 	FileSize          int64      `json:"fileSize"`
+	Checksum          string     `json:"checksum"`
 	StoragePath       string     `json:"storagePath"`
 	DurationSeconds   int        `json:"durationSeconds"`
 	ErrorMessage      string     `json:"errorMessage"`
@@ -111,6 +112,7 @@ func toBackupRecordSummary(item *model.BackupRecord) BackupRecordSummary {
 		Status:            item.Status,
 		FileName:          item.FileName,
 		FileSize:          item.FileSize,
+		Checksum:          item.Checksum,
 		StoragePath:       item.StoragePath,
 		DurationSeconds:   item.DurationSeconds,
 		ErrorMessage:      item.ErrorMessage,