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- plakar-s3-data.md and plakar-incus-integration.md marked SUPERSEDED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>

ks4/local-backup-cron.md

@@ -1,28 +1,23 @@
-# local backup cron on ks4 (leg 1) — re-enabled
+# local backup cron on ks4 (leg 1)
 
-Done 2026-08-22. Context: **both** ks4 backup crons had been commented
-out — no backup of any kind was running.
+Enabled 2026-08-22 (both ks4 backup crons had been commented out — no
+backup of any kind was running). Overview of all legs:
+[../backup-strategy.md](../backup-strategy.md).
 
 ## What runs now (root crontab on ks4)
 
 ```cron
 0 1 * * * /root/scripts/incus-copy.sh -p backup -s backup >> /var/log/incus-copy.log 2>&1
-0 4 * * * /root/scripts/incus-backup.sh -d 164.132.173.57 -u rsyncbackup -i /root/.ssh/id_rsyncbackup -p 2233 -f /root/scripts/incus-backup.db -s data >> /var/log/incus-backup.log 2>&1
 ```
 
 - `01:00` — **local leg**: `incus copy --refresh` of all instances
   into project `backup` on the sdb5 `backup` zpool (replaces the old
-  push to ks2; see [incus-copy.md](incus-copy.md)).
-- `04:00` — **stopgap**: `incus-backup.sh` rsync to ks2 kept alive
-  (manifest pruned of `spot`) until the plakar S3 leg replaces it —
-  then this line goes away as part of
-  [decommissioning ks2](../ks2/plan.md). ⚠️ keep
-  `scripts/incus-backup.db` in this repo in sync with
-  `/root/scripts/incus-backup.db` on ks4.
-
-Both were also run once by hand (in a `screen`) on 2026-08-22 to
-refresh the backups immediately. The old push-to-ks2 copy line stays
-commented out — replica refreshes to ks2 are retired.
+  push to ks2; see [incus-copy.md](incus-copy.md)). The S3 instance
+  leg ([restic-backup.md](restic-backup.md) §6) chains onto this same
+  line once seeded — it reads the replicas this job just refreshed.
+- The rsync stopgap to ks2 (`incus-backup.sh`, 04:00) and the plakar
+  data leg (04:30) are both **retired**; S3 backups are restic's job
+  at 05:00.
 
 ## Cleanup done the same day