fix: set data_directory to canonical path to resolve pgBackRest [058]

pgBackRest stanza-create compares the live cluster's data_directory
against the DataDir stored in pg_control. initdb resolves symlinks
internally so pg_control stores /data/pgdata, but postgresql.conf
explicitly set data_directory to /var/lib/postgresql/data (the symlink),
causing a path mismatch and [058] error on every enable attempt.

Patch data_directory to /data/pgdata immediately after deploying the
postgresql.conf template, scoped to the debpkg_mode/nixpkg_mode block
where the /data volume and symlink are always present. Docker images and
Nix test environments are unaffected as they do not use this Ansible path.

Author: Crispy1975

ansible/tasks/setup-postgres.yml

@@ -167,6 +167,12 @@
       loop_control:
         loop_var: 'pg_config_item'
 
+    - name: Set data_directory to canonical path on data volume
+      ansible.builtin.lineinfile:
+        path: '/etc/postgresql/postgresql.conf'
+        regexp: "^data_directory = '/var/lib/postgresql/data'"
+        line: "data_directory = '/data/pgdata'"
+
     - name: Move read-replica.conf file to /etc/postgresql-custom/read-replica.conf
       ansible.builtin.copy:
         dest: '/etc/postgresql-custom/read-replica.conf'