restic-backup: §7 becomes a scripted, repeatable restore drill

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

ks4/restic-backup.md

@@ -248,15 +248,26 @@ freshness reasoning as before.
 One shared repo would dedup the Ubuntu base across all replicas. Restore granularity: per-file; instance definitions ride
 the dump tree (`incus/<inst>.yaml` + `incus-global-db.sql`).
 
-## 7. Restore test (gate)
+## 7. Restore drill (gate, then quarterly)
 
-Per repo, one restore + diff (never to `/tmp` — 16 GiB tmpfs):
+Scripted — [`restic-restore-test.sh`](https://git.lutran.fr/julien/scripts/src/branch/main/restic-restore-test.sh):
 
 ```sh
-restic -r s3:…/restic-data restore latest --target /backup/restore-test --include '<subtree>'
+/root/scripts/restic-restore-test.sh        # tree + dump, ~2 min
-diff -r … && rm -rf /backup/restore-test
+/root/scripts/restic-restore-test.sh -d     # also loads the dump into a
+                                            # throwaway container (strongest)
 ```
 
+It restores a filesystem tree from the latest snapshot and diffs it
+against the live one, restores a database dump and checks it is
+well-formed (`CREATE TABLE` count + `Dump completed` marker), then
+cleans up (`-k` keeps the files, `-w` changes the work dir). Never
+restore into `/tmp` — it is a 16 GiB tmpfs on ks4; the script writes
+under `/backup`.
+
+A backup leg that has never been restored from is a hypothesis, not a
+backup: run this at the cutover gate and once a quarter after.
+
 ## 8. Cutover (done) + what is left
 
 Legacy on the box: plakar is still installed and its `plakar-data`