docs: rewrite disaster-recovery.md following the full standing process in order

Live-verified against real infrastructure: Kopia's real repo/disk,
PBS's real datastore path, os_update role's real batch/abort behavior
against the actual Ansible code, and the real tag-driven
roles_metadata.yml mechanism behind Layer 1's rebuild claim. Caught a
real KB staleness in the process (PBS datastore usage, 3 weeks stale)
and fixed it properly - removed the point-in-time number entirely
rather than refreshing it, since a data-snapshot doesn't belong in
Trilium at all per the existing content-philosophy rule. Replaced
both mermaid diagrams with real branch/decoupling-shaped Nextcloud
diagrams. All 3 source KB notes stamped.

assets/disaster-recovery-layers.svg

@@ -0,0 +1,4 @@
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