test(infra): verify-notifications smoke + CI job for the OZ→NRC path (refs #56)

make verify-notifications brings the stack up, seeds a published BIG zaaktype, and
asserts a zaak-create notification is delivered to a webhook-sink abonnement. The
sink + driver run as containers inside the compose network and reach OpenZaak/NRC by
container IP (the runner can't reach published ports, and a single-label host isn't
URL-valid). The sink enforces a bearer token because NRC refuses an unauthenticated
callback. New 'notifications' Gitea Actions job runs it (Docker-only).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

infra/notification-sink.py

@@ -0,0 +1,39 @@
+#!/usr/bin/env python3
+"""A throwaway webhook sink for verifying the OpenZaak → NRC notification path.
+
+NRC delivers abonnement callbacks here as POSTs; each body is printed to stdout
+(prefixed `NOTIFICATION `) so the verify harness can assert on `docker logs`.
+
+NRC refuses to register an abonnement whose callback is unauthenticated
+(`no-auth-on-callback`): when validating it sends a probe and expects the callback
+to reject a request without the configured `Authorization` value. So this sink
+enforces that header (EXPECTED_AUTH env) — 401 without it, 204 with it.
+
+Stdlib only. Listens on :9000. See infra/verify-notifications.sh / S-01-c (#56).
+"""
+import http.server
+import os
+import sys
+
+EXPECTED_AUTH = os.environ.get("EXPECTED_AUTH", "Bearer notification-sink-token")
+
+
+class Handler(http.server.BaseHTTPRequestHandler):
+    def do_POST(self):
+        length = int(self.headers.get("content-length", 0))
+        body = self.rfile.read(length).decode("utf-8", "replace")
+        if self.headers.get("Authorization") != EXPECTED_AUTH:
+            self.send_response(401)
+            self.end_headers()
+            return
+        print("NOTIFICATION " + body, flush=True)
+        self.send_response(204)
+        self.end_headers()
+
+    def log_message(self, *args):  # silence default request logging
+        pass
+
+
+if __name__ == "__main__":
+    http.server.HTTPServer(("0.0.0.0", 9000), Handler).serve_forever()
+    sys.exit(0)