doc: split per-host READMEs, gitea cross-repo links, consistency pass

- nuc/README.md and ks4/README.md carry the host sections (+ network
  flows) that lived in the top-level README; links rebased
- top README: repo links (doc/scripts on git.lutran.fr), index points
  at the new per-host pages
- cross-repo references now use https://git.lutran.fr/julien/scripts
  instead of relative ../scripts paths that resolve nowhere
- plakar-s3-data.md and plakar-incus-integration.md marked SUPERSEDED
  / RETIRED with pointers to restic-backup.md; their measurements and
  rationale kept
- install.md, local-backup-cron.md, incus-copy.md: crontab sections
  updated to the live schedule (01:00 replicas, 05:00 restic, Sun
  maintenance); retired legs labelled as such
- restic-backup.md: status live, cutover recorded, post-GC memory
  estimate, seed plan dated
- seafile-gc.md: online GC noted, stale 'crons commented out' removed
- ks2/: what-ks2-does-today rewritten (nothing writes to it any more),
  legs table and gates reflect restic, decommission steps updated
- db-exclude replaces the plakar-era config name (script keeps a
  fallback)

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

ks2/decommission.md

@@ -6,8 +6,8 @@ few days of margin.
 
 ## Preconditions (the plan's release gates)
 
-- [ ] plakar S3 leg running nightly for ≥ a week, `done (rc=0)`,
-      restore test passed ([plakar-s3-data.md](../ks4/plakar-s3-data.md) §7)
+- [ ] restic S3 leg running nightly for ≥ a week, `done (rc=0)`,
+      restore test passed ([restic-backup.md](../ks4/restic-backup.md) §7)
 - [ ] nuc pull leg seeded and one instance test-restored
       ([nuc-seed.md](nuc-seed.md))
 - [ ] local leg (sdb5) cron green in `/var/log/incus-copy.log`
@@ -15,7 +15,8 @@ few days of margin.
 ## 1. Cut the last flows to ks2 (root on ks4)
 
 ```sh
-crontab -e   # delete the 04:00 incus-backup.sh line (164.132.173.57)
+crontab -e   # remove any remaining ks2 line (the 02:00 push, if it was
+             # re-enabled; the 04:00 incus-backup.sh line is long gone)
 incus remote list                 # if a "ks2" remote is still defined:
 incus remote remove ks2
 # drop the rsyncbackup SSH key if it exists only for ks2:
@@ -25,15 +26,15 @@ ls /root/.ssh/id_rsyncbackup* && rm -i /root/.ssh/id_rsyncbackup*
 ## 2. Last look at ks2 before wiping (root on ks2)
 
 ```sh
-# anything in the rsync tree newer than the plakar cutover?
+# anything in the rsync tree newer than the S3 cutover?
 find /backup/ns3061243 -newermt '2026-08-23' | head
 # pre-2023 dirs (catc, mythoughts, qcm, scripts): instances still
 # exist stopped on ks4 — spot-check nothing unique remains:
 ls /backup/ns3061243/{catc,mythoughts,qcm,scripts}
 ```
 
-Archive anything unique to the plakar dump dir on ks4 (it rides the
-nightly `dumps` snapshot to S3 for free once it stops changing).
+Archive anything unique into the restic dump dir on ks4
+(`/backup/dumps`) — it then rides the nightly S3 snapshot for free.
 
 ## 3. Wipe and terminate
 
@@ -52,9 +53,10 @@ renewal date.
       durability bullet and the index line for `ks2/`s "being
       decommissioned" wording (keep the folder as history)
 - [ ] Retire `scripts/incus-backup.sh` + `incus-backup.db` in the repo
-      (superseded by `scripts/plakar-backup.sh`) — keep for reference,
-      note the retirement in their headers
-- [ ] `login` container cleanup parked in
-      [plakar-s3-data.md](../ks4/plakar-s3-data.md): `dropdb outline`,
-      remove native postgres
+      (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)
 - [ ] Mark the ks2 line in `plan.md` done, date it