fix: make mail account creation idempotent by ignoring 'already exists' error

Replace fragile file-content lookup with proper failed_when that accepts
'already exists' exit code 1 as a non-failure. Simpler and works on every run.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

roles/tools/tasks/main.yml

@@ -131,11 +131,6 @@
   delay: 10
   until: postfix_status.rc == 0
 
-- name: Check postfix-accounts.cf exists
-  ansible.builtin.stat:
-    path: "{{ tools_root }}/mailserver/config/postfix-accounts.cf"
-  register: postfix_accounts_file
-
 - name: Create mail accounts
   ansible.builtin.command: >
     docker exec mailserver setup email add {{ item.address }} {{ item.password }}
@@ -143,9 +138,11 @@
     - { address: "noreply@{{ domain_base }}", password: "{{ mailserver_noreply_password }}" }
     - { address: "admin@{{ domain_base }}",   password: "{{ mailserver_admin_password }}" }
     - { address: "jack@{{ domain_base }}",    password: "{{ mailserver_jack_password }}" }
-  when: >
-    not postfix_accounts_file.stat.exists or
-    item.address not in (lookup('file', tools_root + '/mailserver/config/postfix-accounts.cf', errors='ignore') | default('', true))
+  register: mail_account_result
+  changed_when: mail_account_result.rc == 0
+  failed_when: >
+    mail_account_result.rc != 0 and
+    'already exists' not in mail_account_result.stderr
 
 # ── DKIM ─────────────────────────────────────────────────────────────────────
 - name: Check if DKIM key exists