feat(backup): add restic backup role with encrypted incremental snapshots

Restic-backed backups using a many-to-many repository/job model.

- repository = destination: restic repository URL verbatim (s3, local,
  removable device), backend credentials via env passthrough, retention
  policy, and an optional per-repository ntfy notify channel
- job = source set: paths, sqlite and postgres dumps (consistent .backup /
  pg_dump instead of torn live-file copies), pushed to one or more repos
- systemd timer per job, per-repo + per-job flock serialisation, restic
  check with read_data_subset for rotating integrity verification

Service roles declare what to back up (ntfy_backup_*, uptime_kuma_backup_*);
the inventory declares where.

roles/backup/templates/timer-schedule-override.conf.j2

@@ -0,0 +1,6 @@
+# Managed by Ansible (roles/backup). Per-job schedule override for {{ job.name }}.
+[Timer]
+# Reset the inherited value first: OnCalendar is additive, so without an empty
+# assignment the job would fire on both the default and the override schedule.
+OnCalendar=
+OnCalendar={{ job.schedule }}