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- plakar-s3-data.md and plakar-incus-integration.md marked SUPERSEDED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

new-container.md

@@ -75,7 +75,7 @@ them forces a full re-send of its whole disk every night.
 
 **Manual, one line:** data that lives in the filesystem (uploads,
 repositories, mail spools…) must be listed in
-`/root/scripts/restic-paths`, one absolute host path per line:
+[`restic-paths`](https://git.lutran.fr/julien/scripts/src/branch/main/restic-paths), one absolute host path per line:
 
 ```
 /var/lib/incus/storage-pools/data/containers/myservice/rootfs/var/lib/myservice
@@ -84,7 +84,7 @@ repositories, mail spools…) must be listed in
 (The container's `/x` is `…/containers/<name>/rootfs/x` on the host.)
 No comments in that file — every line is read as a path. Regenerable
 caches can be skipped by adding a pattern to
-`/root/scripts/restic-exclude`. Both files live in the `scripts` git
+[`restic-exclude`](https://git.lutran.fr/julien/scripts/src/branch/main/restic-exclude). Both files live in the `scripts` git
 repo: edit there, commit, `git pull` on ks4.
 
 For the database auto-discovery to work, keep the defaults:
@@ -98,8 +98,8 @@ For the database auto-discovery to work, keep the defaults:
   "postgres" (official images do).
 
 A database you do *not* want dumped (huge, static, re-importable)
-goes into `/root/scripts/plakar-db-exclude` as `<instance>/<db>` —
-every skip is logged, so the list cannot rot silently.
+goes into `/root/scripts/db-exclude` as `<instance>/<db>` — every
+skip is logged, so the list cannot rot silently.
 
 ## 5. Verify, once