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/sys-svc-certbot/tasks/02_no_webroot.yml

@@ -15,7 +15,7 @@
   copy:
     dest: "{{ CERTBOT_CREDENTIALS_FILE }}"
     content: |
-      dns_{{ CERTBOT_ACME_CHALLENGE_METHOD }}_api_token = {{ CERTBOT_DNS_API_TOKEN }}
+      dns_{{ CERTBOT_ACME_CHALLENGE_METHOD }}_api_token = {{ CLOUDFLARE_API_TOKEN }}
     owner: root
     group: root
     mode: '0600'