feat(drawio,garage): optional Authentik ForwardAuth in front of UIs

Add `*_authentik_forward_auth` + `*_authentik_forward_auth_url` knobs to
both roles. When enabled:

* drawio: traefik attaches a ForwardAuth middleware pointing at the
  authentik embedded outpost; unauthenticated requests get redirected
  to log in and downstream sees X-Authentik-* identity headers.

* garage WebUI: same ForwardAuth wiring, and `AUTH_USER_PASS` is dropped
  from the container env so authentik is the only gate. Tasks now key
  the htpasswd hash workflow off `_garage_webui_htpasswd_active`
  (`webui_enabled AND NOT authentik_forward_auth`); when authentik
  fronts the UI we skip hashing entirely. htpasswd hash is also now
  cached on disk and re-verified via `htpasswd -vbB` so unchanged
  passwords stop showing as `changed=true` on every run.

Both knobs default to `false`, preserving existing htpasswd/plain behaviour.

roles/garage/tasks/main.yml

@@ -26,12 +26,77 @@
     dest: "{{ garage_docker_compose_dir }}/garage.toml"
     mode: '0644'
 
-- name: Generate bcrypt hash for webui password using htpasswd
-  ansible.builtin.shell: |
-    htpasswd -nbBC 10 "{{ garage_webui_username }}" "{{ garage_webui_password }}"
-  register: _garage_webui_password_hash
+- name: Set webui htpasswd activation fact
+  ansible.builtin.set_fact:
+    # htpasswd only runs when the WebUI is enabled AND authentik ForwardAuth
+    # is not handling authentication. When authentik is in front, the
+    # compose template drops AUTH_USER_PASS so no hash is needed.
+    _garage_webui_htpasswd_active: >-
+      {{
+        garage_webui_enabled
+        and not (garage_webui_authentik_forward_auth | default(false))
+      }}
+
+- name: Read cached webui htpasswd hash
+  ansible.builtin.slurp:
+    src: "{{ garage_docker_compose_dir }}/webui.htpasswd"
+  register: _garage_webui_htpasswd_cached
+  failed_when: false
   changed_when: false
-  when: garage_webui_enabled
+  when: _garage_webui_htpasswd_active
+
+- name: Verify cached webui htpasswd hash still matches password
+  ansible.builtin.command:
+    argv:
+      - htpasswd
+      - -vbB
+      - "{{ garage_docker_compose_dir }}/webui.htpasswd"
+      - "{{ garage_webui_username }}"
+      - "{{ garage_webui_password }}"
+  register: _garage_webui_htpasswd_verify
+  failed_when: false
+  changed_when: false
+  no_log: true
+  when:
+    - _garage_webui_htpasswd_active
+    - _garage_webui_htpasswd_cached.content is defined
+
+- name: Generate bcrypt hash for webui password using htpasswd
+  ansible.builtin.command:
+    argv:
+      - htpasswd
+      - -nbBC
+      - "10"
+      - "{{ garage_webui_username }}"
+      - "{{ garage_webui_password }}"
+  register: _garage_webui_password_hash_new
+  changed_when: true
+  when:
+    - _garage_webui_htpasswd_active
+    - (_garage_webui_htpasswd_cached.content is not defined)
+      or (_garage_webui_htpasswd_verify.rc | default(1) != 0)
+
+- name: Persist webui htpasswd hash on disk
+  ansible.builtin.copy:
+    content: "{{ _garage_webui_password_hash_new.stdout }}\n"
+    dest: "{{ garage_docker_compose_dir }}/webui.htpasswd"
+    mode: '0600'
+  when:
+    - _garage_webui_htpasswd_active
+    - _garage_webui_password_hash_new is changed
+
+- name: Load current webui htpasswd hash
+  ansible.builtin.slurp:
+    src: "{{ garage_docker_compose_dir }}/webui.htpasswd"
+  register: _garage_webui_htpasswd_current
+  changed_when: false
+  when: _garage_webui_htpasswd_active
+
+- name: Expose current webui htpasswd hash to template
+  ansible.builtin.set_fact:
+    _garage_webui_password_hash:
+      stdout: "{{ (_garage_webui_htpasswd_current.content | b64decode).strip() }}"
+  when: _garage_webui_htpasswd_active
 
 - name: Create docker-compose file for garage
   template: