refactor(front-stack): introduce sys-stk-front-base and semi-stateless stack; improve coturn role docs

- Extract common HTTPS + Cloudflare + handler bootstrap into new role sys-stk-front-base
- Update sys-stk-front-proxy, web-svc-cdn, web-svc-file, web-svc-html to depend on sys-stk-front-base
- Add new sys-stk-semi-stateless role combining front-base + back-stateless
- Update web-svc-coturn to use sys-stk-semi-stateless and rewrite README/meta with detailed Coturn description
- Unify sys-util-csp-cert README heading

Ref: ChatGPT conversation https://chatgpt.com/share/68d6cea2-3570-800f-acb3-c3277317f17b

roles/sys-stk-front-base/tasks/cloudflare/01_cleanup.yml

@@ -0,0 +1,13 @@
+- name: "Purge everything from Cloudflare cache for domain {{ domain }}"
+  ansible.builtin.uri:
+    url: "https://api.cloudflare.com/client/v4/zones/{{ cf_zone_id }}/purge_cache"
+    method: POST
+    headers:
+      Authorization: "Bearer {{ CLOUDFLARE_API_TOKEN }}"
+      Content-Type: "application/json"
+    body:
+      purge_everything: true
+    body_format: json
+    return_content: yes
+  async: "{{ ASYNC_TIME if ASYNC_ENABLED | bool else omit }}"
+  poll:  "{{ ASYNC_POLL if ASYNC_ENABLED | bool else omit }}"