feat(openzaak): real notification delivery to the BFF webhook (WP-58)

OpenZaak doesn't serve the Notificaties API itself (it's a separate app,
open-notificaties) — standing one up for a real abonnement would triple
this harness for a benefit it doesn't need (exactly one subscriber, this
repo's own BFF). Instead, an opt-in compose overlay adds a celery worker
and points OpenZaak's NotificationsConfig straight at the BFF's webhook
via a zgw_consumers Service; bootstrap-notificaties.sh configures it
idempotently and verify-notificatie.sh proves a real write delivers to
the BFF's audit trail end-to-end.

Verified live: preflight proves the webhook's shared-secret gate both
ways (204/401), a zaak PATCH triggers real celery delivery, and rerunning
both scripts against an already-configured harness stays idempotent.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

backend/openzaak/verify-notificatie.sh

@@ -0,0 +1,63 @@
+#!/usr/bin/env bash
+# WP-58 — proves the "real write -> real webhook delivery" round-trip end-to-end: PATCHes the
+# zaak bootstrap-catalogus.sh seeded (a notified ZRC resource), then polls the BFF's own audit
+# trail (WP-41) for the resulting `zgw:notificatie` row. Requires bootstrap-catalogus.sh and
+# bootstrap-notificaties.sh to have already run.
+set -euo pipefail
+cd "$(dirname "${BASH_SOURCE[0]}")"
+
+[ -f seeded.env ] || { echo "seeded.env missing — run ./bootstrap-catalogus.sh first" >&2; exit 1; }
+# Not `source`d: seeded.env's ZAAKTYPE_LABEL value contains an unquoted space (fine for the
+# line-oriented C# reader it's written for, not valid as sourceable shell).
+ZAAK_URL=$(grep '^ZAAK_URL=' seeded.env | cut -d= -f2-)
+
+BFF_BASE="${BFF_BASE:-http://localhost:5000}"
+CLIENT_ID="bigregister-test"
+SECRET="bigregister-test-secret"
+
+b64url() { openssl base64 -A | tr '+/' '-_' | tr -d '='; }
+jwt() {
+  local header='{"alg":"HS256","typ":"JWT"}'
+  local payload
+  payload=$(printf '{"iss":"%s","iat":%d,"client_id":"%s","user_id":"%s","user_representation":"%s"}' \
+    "$CLIENT_ID" "$(date +%s)" "$CLIENT_ID" "$CLIENT_ID" "verify")
+  local h p signing_input sig
+  h=$(printf '%s' "$header" | b64url)
+  p=$(printf '%s' "$payload" | b64url)
+  signing_input="$h.$p"
+  sig=$(printf '%s' "$signing_input" | openssl dgst -sha256 -hmac "$SECRET" -binary | b64url)
+  printf '%s.%s' "$signing_input" "$sig"
+}
+
+echo "Triggering a real write: PATCH $ZAAK_URL (bijwerken — WP-57 granted zaken.aanmaken"
+echo "for exactly ONE status, so a second status create 403s; a zaak update is the write this"
+echo "client's narrowed scope can repeat)..."
+response=$(curl -sS -X PATCH -H "Authorization: Bearer $(jwt)" -H 'Content-Type: application/json' \
+  -H 'Content-Crs: EPSG:4326' -H 'Accept-Crs: EPSG:4326' \
+  -d "$(printf '{"toelichting":"wp-58 verify %s"}' "$(date -u +%s)")" \
+  -w $'\n%{http_code}' "$ZAAK_URL")
+http_code="${response##*$'\n'}"
+if [[ ! "$http_code" =~ ^2 ]]; then
+  echo "FAILED: zaak PATCH -> $http_code: ${response%$'\n'*}" >&2
+  exit 1
+fi
+echo "  updated"
+
+echo "Waiting for the BFF's audit trail to show the delivered notification..."
+for _ in $(seq 1 30); do
+  if curl -sS -H 'X-Role: admin' "$BFF_BASE/api/v1/admin/audit" \
+      | python3 -c "
+import json, sys
+rows = json.load(sys.stdin)
+found = any(r['action'] == 'zgw:notificatie' and r['resource'] == '$ZAAK_URL' and r['decision'] == 'allow' for r in rows)
+sys.exit(0 if found else 1)
+"; then
+    echo "  delivered: found a zgw:notificatie/allow row for $ZAAK_URL"
+    exit 0
+  fi
+  sleep 2
+done
+
+echo "FAILED: no delivered notification for $ZAAK_URL after 60s. Diagnostics:" >&2
+docker compose -f docker-compose.openzaak.yml -f docker-compose.openzaak.notificaties.yml logs --tail=50 celery >&2
+exit 1