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- plakar-s3-data.md and plakar-incus-integration.md marked SUPERSEDED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

backup-strategy.md

@@ -13,7 +13,15 @@ docs; this page is the map.
 
 plakar was evaluated first and replaced (its nightly cost scaled with
 tree size — [PlakarKorp/plakar#2338](https://github.com/PlakarKorp/plakar/issues/2338));
-the old backup server **ks2 is retired** ([ks2/plan.md](ks2/plan.md)).
+the old backup server **ks2 is being retired** (decommission by
+Sep 30, 2026 — [ks2/plan.md](ks2/plan.md)): it still holds an ageing
+copy of the instances until the nuc leg below takes over.
+
+**Status 2026-08-28**: the local replica leg and the S3 data leg are
+live. The **nuc pull leg is waiting for FTTH** (expected before end of
+September) and the **S3 instance leg is written but not yet seeded** —
+until both land, instances have only the ks4-local replica off the
+live pool.
 
 ## The map
 
@@ -95,4 +103,4 @@ without the passphrase the S3 buckets are unreadable.
 New instances are picked up automatically by the replica and
 instance legs (all instances, opt-out only). Databases are
 auto-discovered. Only **data trees** need one line in
-`/root/scripts/restic-paths` — see [new-container.md](new-container.md).
+[`restic-paths`](https://git.lutran.fr/julien/scripts/src/branch/main/restic-paths) — see [new-container.md](new-container.md).