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- plakar-s3-data.md and plakar-incus-integration.md marked SUPERSEDED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

@@ -0,0 +1,28 @@
+# Homelab: nuc
+
+Incus host on the LAN.
+
+- Debian 13, Intel Alder Lake-N (iGPU `i915`, shared by both Jellyfin containers) —
+  bare-metal reinstall: [nuc-install.md](nuc-install.md)
+- Instances are bridged onto the LAN (192.168.0.0/24)
+- USB 4 TB WD Red: ZFS pool `usb4t` — `usb4t/backup` → `/backup`
+  (incus exports, `nuc/` + `ks4/` subdatasets, 1 TB quota) and
+  `usb4t/media` → `/srv/media` (media library, shared into containers
+  via `shift=true` disk devices; works because ZFS ≥ 2.2 supports
+  idmapped mounts)
+- Backups: all local instances replicated to the USB pool
+  (`/root/scripts/incus-copy.sh -p backup -s nucbackup`; replicas
+  stopped, autostart off) — see
+  [nuc-install.md](nuc-install.md); ks4 replicas pulled into
+  pool `ks4backup` — see [ks4/incus-copy.md](../ks4/incus-copy.md)
+
+## Instances
+
+| Name | IP | Doc | Features |
+|---|---|---|---|
+| [jellyfin-server](jellyfin-server.md) | 192.168.0.5 | ✅ | unprivileged, autostart; iGPU render node (`gpu` device, render gid) for QSV/VAAPI transcoding; `/srv/media` disk device (`shift=true`); proxy device → host :8096 |
+| [jellyfin-client](jellyfin-client.md) | 192.168.0.6 | ✅ | **privileged**, autostart; full iGPU (`gpu` device, gid 44) → HDMI kiosk (cage + Jellyfin Media Player); custom `raw.lxc` (bind `/dev/snd`, `/dev/input`, host `/run/udev`); Pioneer USB audio as ALSA default; FR keymap; go-librespot Spotify Connect ("Pioneer A-70") |
+| [transmission-bt](transmission-bt.md) | 192.168.0.7 | ✅ | unprivileged, autostart; always-on WireGuard full tunnel → ks4 (egress = 193.70.35.17, kill switch: no default route); `/srv/media` disk device (`shift=true`), downloads to `/media/downloads`; web UI :9091 (LAN only) |
+| blocky | 192.168.0.254 | — | unprivileged, autostart; DNS ad-blocker |
+| privoxy | 192.168.0.11 | — | unprivileged, autostart; filtering HTTP proxy |
+| homeassistant | (stopped) | — | **virtual machine**, 50 GiB root disk on pool `data` |