SearXNG: define the gluetun+searxng+valkey trio as quadlet units

Converts the podman-compose SearXNG stack to boot-persistent quadlet units so
it survives a reboot (the compose deployment did not). searxng sets
Network=gluetun.container, which makes it reuse gluetun's network namespace so
outbound search traffic exits the WireGuard tunnel; it publishes no port of its
own (the port lives on gluetun). gluetun and valkey join a pinned searxng
network (10.89.2.0/24, DNS-enabled) so searxng reaches valkey by name.

Secrets stay in /opt/searxng/.env (mode 0600), referenced via [Service]
EnvironmentFile= so ${VAR} expands. Adds a bats seam asserting the netns
wiring, port placement, boot install, and that no secret material is tracked.

dankclaw/searxng/quadlet/searxng.network

@@ -0,0 +1,16 @@
+# Quadlet network: private bridge for the SearXNG stack (rootless).
+#
+# Install path on dankclaw: ~/.config/containers/systemd/searxng.network
+# Unit name: searxng-network.service (podman network name: systemd-searxng)
+#
+# DNS is enabled by default for quadlet networks. This matters: the automatic
+# "podman" default network has dns_enabled=false, so container names would not
+# resolve. searxng (which reuses gluetun's netns) reaches valkey by the name
+# "valkey" on this network.
+#
+# The subnet is pinned so the value stays stable across reboots. It must match
+# an entry in shared/searxng/limiter.toml trusted_proxies. 10.89.2.0/24 is free
+# (gitea uses 10.89.0.0/24; the old compose net used 10.89.1.0/24).
+
+[Network]
+Subnet=10.89.2.0/24