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/model/backup_record.go

@@ -17,6 +17,7 @@ type BackupRecord struct {
 	Status               string        `gorm:"size:20;index;not null" json:"status"`
 	FileName             string        `gorm:"column:file_name;size:255" json:"fileName"`
 	FileSize             int64         `gorm:"column:file_size;not null;default:0" json:"fileSize"`
+	Checksum             string        `gorm:"column:checksum;size:64" json:"checksum"`
 	StoragePath          string        `gorm:"column:storage_path;size:500" json:"storagePath"`
 	StorageUploadResults string        `gorm:"column:storage_upload_results;type:text" json:"-"`
 	DurationSeconds      int           `gorm:"column:duration_seconds;not null;default:0" json:"durationSeconds"`