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-semi-stateless/README.md

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+# Semi-Stateless Stack (Front + Back) ⚡
+
+## Description
+**sys-stk-semi-stateless** combines the front and back layer into a lightweight, mostly stateless web service stack:
+- Front bootstrap via `sys-stk-front-base` (HTTPS base, optional Cloudflare, handlers)
+- Backend via `sys-stk-back-stateless` (no persistent volumes/DB)
+
+Ideal for services that need TLS/front glue but no database (e.g., TURN/STUN, gateways, simple APIs).
+
+## Responsibilities
+- Prepare the front layer (HTTPS / handlers / optional Cloudflare)
+- Deploy the stateless backend (typically via Docker Compose)
+- Keep domain variables (`domain`) and app-scoped variables (`application_id`) clearly separated

roles/sys-stk-semi-stateless/meta/main.yml

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+galaxy_info:
+  author: "Kevin Veen-Birkenbach"
+  description: "Combined semi-stateless app stack: front bootstrap + stateless backend."
+  license: "Infinito.Nexus NonCommercial License"
+  license_url: "https://s.infinito.nexus/license"
+  company: |
+    Kevin Veen-Birkenbach
+    Consulting & Coaching Solutions
+    https://www.veen.world
+  min_ansible_version: "2.9"
+  platforms:
+    - name: Archlinux
+      versions:
+        - rolling
+  galaxy_tags:
+    - nginx
+    - https
+    - stateless
+    - backend
+    - cloudflare
+    - automation
+  repository: "https://s.infinito.nexus/code"
+  issue_tracker_url: "https://s.infinito.nexus/issues"
+  documentation: "https://docs.infinito.nexus/"

roles/sys-stk-semi-stateless/tasks/main.yml

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+# run_once_sys_stk_full_stateless: deactivated
+
+- name: "sys-stk-front-base"
+  include_role:
+    name: sys-stk-front-base
+  vars:
+    domain:   "{{ domains | get_domain(application_id) }}"
+
+- name: "For '{{ application_id }}': Load sys-stk-back-stateless"
+  include_role:
+    name: sys-stk-back-stateless