Extract restic repo init to shared task file

Deduplicate nearly identical init logic from backend.yml and
backend_sftp.yml into init.yml. Also fixes missing set -euo pipefail
in the sftp backend variant.

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

roles/restic/tasks/backend.yml

@@ -8,12 +8,5 @@
     mode: "0700"
   when: restic_repo is defined and restic_repo.startswith('/')  # only local path
 
-- name: Ensure restic repo is initialized
-  ansible.builtin.shell: |
-    set -euo pipefail
-    source /etc/restic/restic.env
-    restic snapshots > /dev/null 2>&1 || restic init
-    touch /etc/restic/.initialized
-  args:
-    creates: /etc/restic/.initialized
+- ansible.builtin.include_tasks: init.yml