doc: plakar out of the current architecture, kept as reference

backup-strategy.md now leads with the three target legs (local incus
push to sdb, nuc pull over WireGuard, restic to S3 for DBs and
selected trees) and carries the plakar/ks2 story in a closing History
section. restic-backup.md, install.md, local-backup-cron.md and the
ks2 docs describe the predecessor only as 'the first S3
implementation', with one operational note that plakar is still
installed for issue #2338 reproduction.

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

ks2/decommission.md

@@ -55,8 +55,8 @@ renewal date.
 - [ ] Retire `scripts/incus-backup.sh` + `incus-backup.db` in the repo
       (superseded by [`restic-backup.sh`](https://git.lutran.fr/julien/scripts/src/branch/main/restic-backup.sh)) —
       keep for reference, note the retirement in their headers
-- [ ] `login` container cleanup: the native postgres still holds the
-      pre-migration `outline` DB — `dropdb outline`, remove the
-      package (noted in
-      [plakar-s3-data.md](../ks4/plakar-s3-data.md) §3)
+- [ ] `login` container cleanup: its **native** postgres still holds
+      the pre-migration `outline` database (live Outline uses the
+      dockerised `outline-postgres-1`) — `dropdb outline` and remove
+      the native postgres package
 - [ ] Mark the ks2 line in `plan.md` done, date it