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/traefik/templates/docker-compose.yml.j2

@@ -7,6 +7,7 @@ services:
     ports:
       - "80:80"
       - "443:443"
+      - "993:993"
     volumes:
       - /var/run/docker.sock:/var/run/docker.sock:ro
       - {{ traefik_data_dir }}/traefik.yml:/traefik.yml:ro
@@ -22,7 +23,11 @@ services:
       - "traefik.http.routers.dashboard.tls.certresolver=letsencrypt"
       - "traefik.http.routers.dashboard.service=api@internal"
       - "traefik.http.routers.dashboard.middlewares=dashboard-auth"
-      - "traefik.http.middlewares.dashboard-auth.basicauth.users={{ traefik_dashboard_users }}"
+{% set ns = namespace(entries=[]) %}
+{% for user in traefik_dashboard_users | default([]) %}
+{% set ns.entries = ns.entries + [user.username + ':' + (user.password | password_hash('bcrypt', (user.username | hash('md5'))[:22]) | replace('$', '$$'))] %}
+{% endfor %}
+      - "traefik.http.middlewares.dashboard-auth.basicauth.users={{ ns.entries | join(',') }}"
       # Rate-limit middlewares — referenced by services as rate-limit@docker / rate-limit-lax@docker
       # Standard: 60 req/min per IP, burst 20 — protects all services from bot floods
       - "traefik.http.middlewares.rate-limit.rateLimit.average=60"