SearXNG docs: document quadlet boot-persistence for the trio

Update SearXNG, Gluetun, and Valkey to describe the quadlet units instead of
compose: searxng sets Network=gluetun.container, the units install to
~/.config/containers/systemd/, and scripts/deploy-searxng-quadlet.sh deploys
them. Add a Boot Persistence section to the Stack doc. Correct the Gluetun
tunnel check (the :8000 server 404s; verify tun0 instead) and note the pinned
10.89.2.0/24 subnet.

docs/Dankclaw/Valkey.md

@@ -18,13 +18,15 @@ The official SearXNG docker-compose ships a `searxng-valkey` container. Valkey i
 
 ## Configuration
 
-```yaml
-services:
-  valkey:
-    image: valkey/valkey:9-alpine
-    restart: always
-    volumes:
-      - /opt/searxng/valkey:/data
+The valkey unit is `searxng-valkey.container` (see [[SearXNG]] for the trio). It joins the searxng network and is reachable by the name `searxng-valkey` (container name) and the alias `valkey`:
+
+```ini
+[Container]
+ContainerName=searxng-valkey
+Network=searxng.network
+NetworkAlias=valkey
+Image=docker.io/valkey/valkey:9-alpine
+Volume=/opt/searxng/valkey:/data
 ```
 
 SearXNG connects to it via `settings.yml`:
@@ -34,6 +36,8 @@ valkey:
   url: "valkey://searxng-valkey:6379/0"
 ```
 
+It sets `WantedBy=default.target`. User linger is on, so it starts at boot.
+
 ## Volumes
 
 | Container path | Host path (convention) |