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- plakar-s3-data.md and plakar-incus-integration.md marked SUPERSEDED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

@@ -1,4 +1,15 @@
-# plakar data + DB backups on ks4 (leg 3)
+# plakar data + DB backups on ks4 (leg 3) — SUPERSEDED
+
+> **Historical.** This leg ran 2026-08-22 → 08-28 and was replaced by
+> restic: see **[restic-backup.md](restic-backup.md)**. plakar's
+> incremental cost scaled with tree size rather than churn
+> ([PlakarKorp/plakar#2338](https://github.com/PlakarKorp/plakar/issues/2338)).
+> The `plakar-data` kloset and the plakar install are deliberately kept
+> until that issue concludes (the report offers debug runs against that
+> repository); the `plakar-incus` bucket can be deleted. Kept here for
+> the measurements and the design rationale, which the restic doc
+> builds on.
+
 
 Status: **live since 2026-08-24** — S3 kloset
 (`lutran-ks4-plakar-data`) seeded (dumps + 9 fs sources), restore
@@ -19,7 +30,7 @@ leg 1's job. Direct-to-S3 keeps the incremental behavior — the
 skip-unchanged baseline is the parent snapshot's VFS read from the
 kloset (plakar caches repository metadata locally; v1.1.4's dirpack
 prefetcher targets exactly this remote-backend walk). Driver script:
-[`scripts/plakar-backup.sh`](../../scripts/plakar-backup.sh)
+[`scripts/plakar-backup.sh`](https://git.lutran.fr/julien/scripts/src/branch/main/plakar-backup.sh)
 (flock, incus DB dumps, iterates a sources file, prune + maintenance;
 its `-k` takes any `plakar at` target, so `-k @s3` — a throwaway
 local kloset, e.g. `-k /backup/plakar-test`, works for testing before