fix: valid Caddyfile handle_errors + frontend health-gate in deploys (#20)

The learning-platform (Caddy) container crash-looped since this morning's
89d3395: `Content-Type application/json` is not a valid subdirective of
`respond` — a Caddyfile parse error aborts Caddy at startup, taking the
whole app offline for authenticated users while every CI run stayed green.
Verified with caddy v2.10.0: "unrecognized subdirective 'Content-Type',
at Caddyfile:53"; the fixed file validates clean.

CI was blind to this twice over: test.yml swaps the real Caddyfile for
Caddyfile.test in the test image, and the deploy health-gate from #18
only covers PocketBase.

- Caddyfile: set Content-Type via a `header @api` directive before the
  `respond`; behaviour of the JSON error response is unchanged
- infra/*/site/deploy-playbook.yml: frontend health-gate (docker exec
  wget --spider on the container) with log dump + abort on failure, and
  an always-on post-deploy smoke report (compose ps, proxy health,
  team_members auth-methods, PocketBase log tail) for visibility behind
  the auth perimeter

Closes #20

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

infra/production/site/deploy-playbook.yml

@@ -100,3 +100,53 @@
           {{ pb_logs.stdout | default('') }}
           {{ pb_logs.stderr | default('') }}
       when: pb_health is failed
+
+    # The frontend container carries the SPA + Caddy. An invalid Caddyfile
+    # crash-loops it at startup while the PocketBase gate stays green — that
+    # outage (issue #20) was invisible to CI because the test job swaps in
+    # Caddyfile.test. Gate on the real container actually serving.
+    - name: Wait for the frontend (Caddy) to become healthy
+      ansible.builtin.command: docker exec learning-platform wget -q --spider http://127.0.0.1:80/
+      register: fe_health
+      until: fe_health.rc == 0
+      retries: 18
+      delay: 5
+      changed_when: false
+      ignore_errors: yes
+
+    - name: Collect frontend logs for diagnosis
+      ansible.builtin.command: docker logs --tail 80 learning-platform
+      register: fe_logs
+      when: fe_health is failed
+      changed_when: false
+      failed_when: false
+
+    - name: Abort deploy — frontend is not healthy
+      ansible.builtin.fail:
+        msg: |
+          The frontend (Caddy) container did not become healthy after the deploy.
+          Recent container logs:
+          {{ fe_logs.stdout | default('') }}
+          {{ fe_logs.stderr | default('') }}
+      when: fe_health is failed
+
+    # Observability behind the auth perimeter: surface the end-to-end state
+    # in the CI log on every deploy.
+    - name: Post-deploy smoke report
+      ansible.builtin.shell: |
+        cd /opt/learning-platform
+        echo '--- docker compose ps ---'
+        docker compose ps
+        echo '--- frontend -> pocketbase proxy health ---'
+        docker exec learning-platform wget -q -O - http://127.0.0.1:80/api/health || echo 'PROXY HEALTH FAILED'
+        echo '--- team_members auth methods (OIDC provider present?) ---'
+        docker exec pocketbase-learning wget -q -O - http://127.0.0.1:8090/api/collections/team_members/auth-methods || echo 'AUTH-METHODS FAILED'
+        echo '--- pocketbase logs (tail 30) ---'
+        docker compose logs --tail=30 pocketbase-learning
+      register: smoke_report
+      changed_when: false
+      failed_when: false
+
+    - name: Show smoke report
+      ansible.builtin.debug:
+        msg: "{{ smoke_report.stdout_lines }}"