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- plakar-s3-data.md and plakar-incus-integration.md marked SUPERSEDED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

nuc/nuc-install.md

@@ -19,7 +19,7 @@ How to rebuild the Incus host from scratch if `/dev/sda` (512 GB SSD,
 The ZFS pool `data` lives on `sda5` → **all instance root disks are lost**
 with the OS. The USB pool `usb4t` (backups + media) survives — all
 instances are replicated onto it with
-[`scripts/incus-copy.sh`](../scripts/incus-copy.sh) (deployed at
+[`scripts/incus-copy.sh`](https://git.lutran.fr/julien/scripts/src/branch/main/incus-copy.sh) (deployed at
 `/root/scripts/` on nuc; set up 2026-08-09):
 
 ```sh