refactor(dns): unify Cloudflare + Hetzner handling across roles

- replaced CERTBOT_DNS_API_TOKEN with CLOUDFLARE_API_TOKEN everywhere
- introduced generic sys-dns-cloudflare-records role for managing DNS records
- added sys-dns-hetzner-rdns role with both Cloud (hcloud) and Robot API flavors
- updated Mailu role to:
  - generate DKIM before DNS setup
  - delegate DNS + rDNS records to the new generic roles
- removed legacy per-role Cloudflare vars (MAILU_CLOUDFLARE_API_TOKEN)
- extended group vars with HOSTING_PROVIDER for rDNS flavor decision
- added hetzner.hcloud collection to requirements

This consolidates DNS management into reusable roles,
supports both Cloudflare and Hetzner providers,
and standardizes variable naming across the project.

roles/srv-proxy-6-6-domain/tasks/01_cloudflare.yml

@@ -16,7 +16,7 @@
     url: "{{ cf_api_url }}?name={{ domain | to_primary_domain }}"
     method: GET
     headers:
-      Authorization: "Bearer {{ CERTBOT_DNS_API_TOKEN }}"
+      Authorization: "Bearer {{ CLOUDFLARE_API_TOKEN }}"
       Content-Type: "application/json"
     return_content: yes
   register: cf_zone_lookup_dev