WP-30/31/32: backup targets, strategies and execution

SMB (smbclient) and FTP/FTPS (curl with netrc) targets with encrypted
credentials and connection test; strategies with cron schedule, retention,
optional openssl AES-256 encryption; sources: PVC hostpath tar, pg_dumpall
in the Postgres pod, namespace manifests, host directory. Backup job
collects, encrypts, uploads, verifies size, records sha256 and applies
retention on the target; scheduler starts due strategies. Backups page
with target/strategy forms, run now and history. Restore guide in docs.

Co-Authored-By: Claude Fable 5.1 <noreply@anthropic.com>

backend/Cargo.lock

@@ -1100,6 +1100,7 @@ dependencies = [
  "serde",
  "serde_json",
  "sqlx",
+ "tempfile",
  "tokio",
  "uuid",
 ]