docs: document disposable restore rehearsal

.superpowers/sdd/disposable-restore-doc-fix-report.md

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+# Disposable restore documentation fix report
+
+## Scope
+
+Updated `scripts/backup/README.md` only for the documentation change. This
+report is the requested verification artifact.
+
+## Change
+
+The monthly recovery rehearsal now documents a unique disposable Compose
+project with project-labeled volumes, snapshot selection, staging restore,
+PostgreSQL import, restoration of uploads and both TDLib volumes, disposable
+service startup, health and log checks, retained-file validation, known STL
+checksum and metadata comparison, narrowly scoped cleanup, and an evidence
+template.
+
+The guide explicitly warns operators not to use production project names or
+volumes, and states that this documentation update did not run the rehearsal.
+
+## Verification
+
+- Focused text check: passed. Confirmed the guide contains the unique project
+  warning, snapshot selection, staging restore, PostgreSQL import, all three
+  protected volume restores, health check, retained-file validation, checksum
+  comparison, scoped cleanup, evidence template, and the statement that the
+  rehearsal was not run.
+- `git diff --check`: passed.
+- No recovery, Docker, Restic, PostgreSQL, or health-check commands were run;
+  this change documents the operator procedure only.