Add SHA-256 verification of copied backups (v3.3.0)

After copying a backup from Home Assistant, compare its SHA-256 (via
sha256sum on the server) with a hash of the local file. On mismatch,
delete the local copy and send an error notification; the remote backup
is left in place so the run can be retried. If either checksum cannot
be computed, the run fails rather than reporting an unverified copy as
successful.

- Add VERIFY_CHECKSUM config option (default true; "false" skips the check)
- Make boolean config values case-insensitive (VERIFY_CHECKSUM, NTFY_ENABLED,
  DISCORD_ENABLED), so "True" now enables them. Previously only lowercase
  "true" did, so NTFY_ENABLED="True" silently left ntfy off
- Bump version to 3.3.0

home_assistant_backup.conf

@@ -36,3 +36,12 @@ DISCORD_WEBHOOK_URL="DISCORD_WEBHOOK_URL"  # Replace with your Discord webhook U
 # The file must be writable by the user running ha-backup, see the README
 # Set to "none" to disable logging to a file (console output is unaffected)
 LOG_FILE="/var/log/ha-backup.log"
+
+## File Validation
+# Verify the local copy against the backup on the Home Assistant host using
+# SHA-256 (sha256sum on the server, compared with the hash of the local file).
+# If the hashes differ, the local copy is deleted and an error notification is
+# sent; the backup on the Home Assistant server is left untouched so you can retry.
+# Verification reads the whole file twice (once on each side), so it adds time
+# on large backups. Set to "false" to skip it. Accepts true/false in any case.
+VERIFY_CHECKSUM="true"  # "true" (default) or "false"