feat(authentik): split-horizon host rewrite + proxy-app mode/group bindings

* `authentik_host_rewrite_domains`: extra hostnames that reach the
  authentik container but make it generate URLs (OIDC issuer, reset
  links) as if requested from the canonical `authentik_domains[0]`.
  Each entry gets its own traefik router and a URL-based loadbalancer
  service that disables passHostHeader and pins X-Forwarded-Host via
  middleware, so server-to-server calls on internal FQDNs keep traffic
  in the LAN while the iss claim stays aligned with the public host.
  Uses a network alias on the canonical FQDN so traefik (sharing the
  network) resolves the URL upstream to this very container.

* proxy-app blueprint:
  - `mode` (default `forward_single`) lets callers pick between proxy,
    forward_single and forward_domain providers in one template.
  - `allowed_groups`: when set, emit one PolicyBinding per group on
    the application; authentik OR-evaluates bindings, so users in any
    listed group pass and others are denied.

Existing inventories with an empty list see no behavioural change.

roles/authentik/templates/blueprints/blueprint-proxy-app.yaml.j2

@@ -20,6 +20,16 @@ entries:
 
       internal_host: "{{ item.internal_host }}"
       external_host: "{{ item.external_host }}"
+{# Provider mode controls how authentik treats the proxy app:
+   - proxy          : the outpost itself proxies traffic to internal_host
+   - forward_single : a single app behind an external reverse proxy
+                      (traefik forwardauth talks to authentik per-domain)
+   - forward_domain : wildcard mode — one provider guards every host on a
+                      cookie domain; configure forward_auth_mode=domain on
+                      the outpost in that case. Default to forward_single
+                      since that's the common ForwardAuth-with-traefik
+                      pattern. #}
+      mode: {{ item.mode | default('forward_single') }}
 
 {% if item.skip_path_regex is defined and item.skip_path_regex|length > 0 %}
       skip_path_regex: |
@@ -34,3 +44,20 @@ entries:
       name: "{{ item.name | default(item.slug) }}"
       slug: {{ item.slug }}
       provider: !KeyOf proxy-provider-{{ item.slug }}
+
+{% if item.allowed_groups is defined and item.allowed_groups | length > 0 %}
+{# Restrict access to listed groups: one PolicyBinding per group, all bound
+   to the application. Authentik treats multiple bindings on the same target
+   as OR (a user matching any binding passes), and a request from a user in
+   none of the bound groups is denied. #}
+{%   for group_name in item.allowed_groups %}
+  - model: authentik_policies.policybinding
+    identifiers:
+      target: !KeyOf app-{{ item.slug }}
+      order: {{ loop.index0 }}
+      group: !Find [authentik_core.group, [name, "{{ group_name }}"]]
+    attrs:
+      enabled: true
+      negate: false
+{%   endfor %}
+{% endif %}

roles/authentik/templates/docker-compose.yml.j2

@@ -43,12 +43,19 @@ services:
       postgres:
         condition: service_healthy
     networks:
-      - {{ authentik_backend_network }}
-      - {{ authentik_traefik_network }}
+      {{ authentik_backend_network }}: {}
+      # Network alias so traefik (which shares this network) can resolve
+      # the canonical FQDN to this container directly. The URL-based
+      # service below uses that to send upstream traffic with a fixed
+      # Host header equal to the canonical hostname.
+      {{ authentik_traefik_network }}:
+        aliases:
+          - {{ authentik_domains[0] }}
     labels:
       - traefik.enable=true
       - traefik.docker.network={{ authentik_traefik_network }}
       - traefik.http.routers.{{ authentik_service_name }}.rule={% for d in authentik_domains %}Host(`{{ d }}`){% if not loop.last %} || {% endif %}{% endfor +%}
+      - traefik.http.routers.{{ authentik_service_name }}.service={{ authentik_service_name }}
 {% if authentik_use_ssl %}
       - traefik.http.routers.{{ authentik_service_name }}.entrypoints=websecure
       - traefik.http.routers.{{ authentik_service_name }}.tls=true
@@ -59,6 +66,34 @@ services:
       - traefik.http.routers.{{ authentik_service_name }}.entrypoints=web
 {% endif %}
       - traefik.http.services.{{ authentik_service_name }}.loadbalancer.server.port={{ authentik_port }}
+{% if authentik_host_rewrite_domains | length > 0 %}
+      # Server-to-server entry: a separate service points at this very
+      # container by the canonical FQDN (resolved via the network alias
+      # above) and disables passHostHeader so the upstream Host header
+      # becomes `{{ authentik_domains[0] }}`. Authentik builds OIDC issuer
+      # URLs from X-Forwarded-Host (not Host), so we also pin that header
+      # via middleware. Together this keeps the iss claim aligned with
+      # the public hostname browsers see during login, even when the
+      # request itself arrived on an internal *.int.* FQDN.
+      - traefik.http.services.{{ authentik_service_name }}-rewrite.loadbalancer.server.url=http://{{ authentik_domains[0] }}:{{ authentik_port }}
+      - traefik.http.services.{{ authentik_service_name }}-rewrite.loadbalancer.passhostheader=false
+      - traefik.http.middlewares.{{ authentik_service_name }}-xfh-rewrite.headers.customrequestheaders.X-Forwarded-Host={{ authentik_domains[0] }}
+{% for d in authentik_host_rewrite_domains %}
+      - traefik.http.routers.{{ authentik_service_name }}-rewrite-{{ loop.index0 }}.rule=Host(`{{ d }}`)
+      - traefik.http.routers.{{ authentik_service_name }}-rewrite-{{ loop.index0 }}.priority=100
+      - traefik.http.routers.{{ authentik_service_name }}-rewrite-{{ loop.index0 }}.service={{ authentik_service_name }}-rewrite
+      - traefik.http.routers.{{ authentik_service_name }}-rewrite-{{ loop.index0 }}.middlewares={{ authentik_service_name }}-xfh-rewrite
+{% if authentik_use_ssl %}
+      - traefik.http.routers.{{ authentik_service_name }}-rewrite-{{ loop.index0 }}.entrypoints=websecure
+      - traefik.http.routers.{{ authentik_service_name }}-rewrite-{{ loop.index0 }}.tls=true
+{% if traefik_cert_mode | default('selfsigned') == 'acme' %}
+      - traefik.http.routers.{{ authentik_service_name }}-rewrite-{{ loop.index0 }}.tls.certresolver={{ traefik_ssl_cert_resolver | default('dns') }}
+{% endif %}
+{% else %}
+      - traefik.http.routers.{{ authentik_service_name }}-rewrite-{{ loop.index0 }}.entrypoints=web
+{% endif %}
+{% endfor %}
+{% endif %}
 
   worker:
     image: {{ authentik_image }}