Add Dovecot IMAP + Fetchmail, fix Gitea SSO, simplify credential management

- Add local Dovecot IMAP server exposed via Traefik IMAPS on port 993;
  Roundcube now connects to it internally instead of requiring manual server entry
- Add Fetchmail integration for pulling from external POP3 accounts with
  configurable per-account poll interval
- Fix Gitea SSO registration: DISABLE_REGISTRATION=false + ALLOW_ONLY_EXTERNAL_REGISTRATION
  allows Keycloak-authenticated users to get accounts while blocking public sign-up;
  disable legacy OpenID 2.0 sign-in
- Fix Keycloak post-logout redirect for Nextcloud (valid post logout redirect URI)
- Replace all pre-hashed credentials (Traefik dashboard, registry, Dovecot) with
  plaintext passwords in vault; Ansible generates deterministic bcrypt/SHA-512 hashes
  at deploy time — no more manual htpasswd commands
- Rewrite check-vault.sh with Python/PyYAML to properly validate both scalar and
  list-type secrets
- Update provisioning and configuration runbooks throughout

ansible/roles/registry/tasks/main.yml

@@ -12,10 +12,9 @@
     - "{{ registry_data_dir }}/auth"
   tags: registry
 
-# htpasswd content is stored in vault and deployed as a file
 - name: Deploy htpasswd file
-  ansible.builtin.copy:
-    content: "{{ registry_htpasswd }}\n"
+  ansible.builtin.template:
+    src: htpasswd.j2
     dest: "{{ registry_data_dir }}/auth/htpasswd"
     owner: root
     group: root