Retire Authentik now that every app logs in with Pocket ID

All of Authentik's applications (including Tailscale's tailnet login)
have moved to Pocket ID, and its only recent sign-ins were those
handovers. Drops its stack, the auth. route and DNS record, the auth
subdomain variables, Gotify's leftover hairpin entries, and the dockcheck
exclude; the docs now name Pocket ID as the SSO provider.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

dns.yml

@@ -25,8 +25,6 @@
             ip: "{{ network.netcup_ip }}"
           - name: "social"
             ip: "{{ network.netcup_ip }}"
-          - name: "auth"
-            ip: "{{ network.netcup_ip }}"
           - name: "audio"
             ip: "{{ network.netcup_ip }}"
           - name: "books"
@@ -88,6 +86,8 @@
             ip: "{{ network.netcup_ip }}"
           - name: "sync"  # kosync, replaced by Kavita
             ip: "{{ network.netcup_ip }}"
+          - name: "auth"  # Authentik, replaced by Pocket ID
+            ip: "{{ network.netcup_ip }}"
           - name: "files"
             ip: "{{ network.netcup_ip }}"
           - name: "kanboard"