fix(n8n): close the startup-splash race in the owner bootstrap

On a fresh dedicated database (variant 1) the owner probe hit n8n's startup splash 22s after force-recreate: HTTP 200 text/html, which the probe read as 'owner exists', so owner setup was skipped vacuously and Playwright met the first-run wizard instead of the login form. Wait on /healthz/readiness (503 until the DB is connected and migrations ran) before bootstrapping, and accept only JSON responses in the owner probe - the splash is text/html, so the until-gate turns the previously inert retries into real ones and fails loudly if the splash persists.

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

roles/web-app-n8n/tasks/01_core.yml

@@ -30,7 +30,7 @@
 
 - name: "🔎 Wait for n8n health check to pass"
   uri_retry:
-    url: "{{ N8N_INTERNAL_API_URL }}/healthz"
+    url: "{{ N8N_INTERNAL_API_URL }}/healthz/readiness"
     method: GET
     status_code: 200
     return_content: false

roles/web-app-n8n/tasks/02_bootstrap.yml

@@ -26,6 +26,7 @@
     status_code: [200, 401, 403]
     return_content: false
   register: n8n_owner_probe
+  until: "'json' in (n8n_owner_probe.content_type | default(''))"
   retries: 4
   delay: 30