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- plakar-s3-data.md and plakar-incus-integration.md marked SUPERSEDED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

@@ -1,7 +1,7 @@
 # incus-copy — ks4 instance replication
 
 Incremental replication of all ks4 Incus instances via
-[`scripts/incus-copy.sh`](../scripts/incus-copy.sh)
+[`scripts/incus-copy.sh`](https://git.lutran.fr/julien/scripts/src/branch/main/incus-copy.sh)
 (`incus copy --refresh`: ZFS-incremental, produces ready-to-start
 replicas instead of tarballs). Two legs replace the historical nightly
 push to `ks2` (decommissioning):
@@ -35,7 +35,7 @@ incus-copy.sh -p <dest-project> -s <dest-pool>                         # local
   `--refresh-exclude-older` keeps expired snapshots from being re-sent.
 - Consistency: copies of running instances are crash-consistent;
   databases may need recovery on restore. Snapshot history mitigates,
-  and [`incus-backup.sh`](../scripts/incus-backup.sh) (proper
+  and [`incus-backup.sh`](https://git.lutran.fr/julien/scripts/src/branch/main/incus-backup.sh) (proper
   `mariadb-dump`s + selected paths, manifest-driven) covers the DBs.
 - **VMs**: `--refresh` re-sends a VM's **full block volume** every run
   unless source and replica share a common snapshot — containers diff
@@ -63,7 +63,9 @@ Cron (root on ks4) — replaces both ks2 jobs:
 
 ```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 -f /root/scripts/incus-backup.db -s data >> /var/log/incus-backup.log 2>&1
+# retired 2026-08-28 — dumps + data trees are restic's job (05:00),
+# see ../backup-strategy.md and restic-backup.md
+#0 4 * * * /root/scripts/incus-backup.sh -f /root/scripts/incus-backup.db -s data >> /var/log/incus-backup.log 2>&1
 ```
 
 (`incus-backup.sh` without `-d` now writes locally to