feat(backup): Add restic backup

Restic backup creates a snapper snapshot of the root system which it
then chroots into and starts a restic backup to a (wasabi) S3 bucket to.

Intended to roughly follow this
<https://codeberg.org/silmaril/my-restic-solution> solution to achieve
restic backup of the _newest_ snapshot of my live root system.

roles/backup/tasks/main.yaml

@@ -2,4 +2,10 @@
   import_tasks: snapper.yaml
   tags:
     - btrfs
-    - snapshots
+    - snapper
+
+- name: Set up restic backups
+  import_tasks: restic.yaml
+  tags:
+    - btrfs
+    - restic

roles/backup/tasks/restic.yaml

@@ -0,0 +1,52 @@
+- name: Install restic
+  community.general.xbps:
+    name:
+      - restic
+    state: present
+  tags: packages
+
+- name: Ensure restic configuration directory exists
+  ansible.builtin.file:
+    dest: "/etc/restic/root"
+    state: directory
+
+- name: Create restic root backup configuration
+  ansible.builtin.template:
+    src: restic.conf.j2
+    dest: "/etc/restic/root/restic.conf"
+    mode: 0600
+    force: true # ensure contents are always exact
+
+- name: Ensure files and exclude files exist # Only change if necessary
+  ansible.builtin.file:
+    dest: "/etc/restic/root/{{ item }}"
+    state: touch
+    modification_time: preserve
+    access_time: preserve
+  loop:
+    - include
+    - exclude
+
+- name: Create include files # TODO: Rename file to include?
+  ansible.builtin.copy:
+    content: "/"
+    dest: "/etc/restic/root/include"
+    force: true # ensure contents are always exact
+
+- name: Install snapstic script
+  ansible.builtin.copy:
+    src: snapstic
+    dest: "/usr/bin/snapstic"
+    mode: 0744
+
+- name: Ensure restic configuration directory exists
+  ansible.builtin.file:
+    dest: "/etc/cron.weekly"
+    state: directory
+
+- name: Add snapstic to weekly cronjobs
+  ansible.builtin.copy:
+    content: "#!/bin/sh\n\nsnapstic backup"
+    dest: "/etc/cron.weekly/snapstic"
+    mode: 0755
+    force: true # ensure contents are always exact