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- plakar-s3-data.md and plakar-incus-integration.md marked SUPERSEDED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

@@ -21,8 +21,8 @@ current state → new library → delete old → GC `-r`, which works for
 takes the dataset to ~600 k inodes, at which point per-file backup of
 seafile becomes ordinary.
 
-All crontab backup jobs on ks4 remain commented out until restart +
-validation completes.
+(The ks4 backup crons were paused during the GC window and are back
+on since 2026-08-27.)
 
 ## Root cause of the 24.9 M-inode / 933 G dataset
 
@@ -73,9 +73,15 @@ setting (the existing per-library 30 d row stays). Users can still
 override per library in the web UI if
 `ENABLE_REPO_HISTORY_SETTING = True` in `seahub_settings.py`.
 
-### 2. GC — offline, maintenance window
+### 2. GC — maintenance window
 
-CE's GC requires seafile stopped. On this pathological store expect
+⚠️ Superseded by experience: with the **MySQL backend seafile runs
+*online* GC** (the runs print "use online GC"), so stopping the
+services is not required — the recurring cron below does not. The
+2026-08-26 run stopped them anyway, out of caution. What follows is
+that run's procedure.
+
+CE's GC traditionally requires seafile stopped. On this pathological store expect
 **hours** (it traverses fs/commits on a cold HDD); the dry-run sizes
 the window. Inside the container:
 
@@ -115,7 +121,7 @@ du -sh .../seafile/rootfs/opt/seafile/seafile-data/storage/blocks   # was 867 G
   0 6 15 1,4,7,10 * incus exec seafile -- sh -c "cd /opt/seafile/seafile-server-latest && ./seaf-gc.sh && ./seaf-gc.sh -r && ./seaf-gc.sh -R" >> /var/log/seafile-gc.log 2>&1
   ```
   At the post-cleanup scale (~120 k live files) the full sequence is
-  minutes, not hours. Keep it clear of the 01:00/04:30/05:00 backup
+  minutes, not hours. Keep it clear of the 01:00/05:00 backup
   slots.
 - 2026-08-27 follow-ups executed: 8 old libraries deleted by user →
   trash emptied → `seaf-gc -r` purged their stores (18 m, blocks now