feat(backup): skip-if-unchanged + scheduled CronJob in the chart

Builds on the dedicated backup (erp#31).

Skip-if-unchanged: each half (DB / documents) carries a content fingerprint at
erp/<env>/.fp-{db,docs} and is dumped+uploaded only if it differs from the last
run — a quiet ERP day re-uploads nothing. Fingerprint = durable BUSINESS content
only: DB = count+max(tms) over tms tables EXCEPT volatile churn (llx_const,
llx_user, session/cron); docs EXCLUDE */temp/* (Dolibarr stats cache) — from both
the fingerprint and the tar. Proven live: 1st run uploads both, immediate 2nd run
skips both (uploaded=0).

Automation: the in-container logic moves to chart/files/backup-job.sh (single
source of truth, read by the orchestrator AND the chart). New
chart/templates/backup-cronjob.yaml renders a daily CronJob + ConfigMap +
VaultStaticSecret, gated by backup.enabled (default false). Helm-verified: off by
default (0 CronJobs), on renders correctly, env-aware (PREFIX erp/prod vs
erp/sandbox), script embedded.

Activation (documented): store GCS HMAC creds at kvv2/<backup.vaultS3Path>
(default erp/backup), grant the erp `auth` Vault role read on it (tools change),
set backup.enabled=true. Until then the orchestrator runs on demand.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

ops/backup/dolibarr-backup.sh

@@ -86,7 +86,7 @@ run_backup() {
   log "Copying GCS creds into a transient secret in $NS (values stay base64)"
   copy_s3_secret
   log "Backup ${ENV}: DB=$DB  PVC=$PVC  ->  s3://$BUCKET/$PREFIX/{db,docs}/"
-  local B64; B64="$(b64 "$(cat "${SCRIPT_DIR}/backup-job.sh")")"
+  local B64; B64="$(b64 "$(cat "${SCRIPT_DIR}/../../chart/files/backup-job.sh")")"
   kubectl delete job dolibarr-backup -n "$NS" --ignore-not-found >/dev/null 2>&1 || true
   kubectl apply -f - >/dev/null <<EOF
 apiVersion: batch/v1