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
