SearXNG quadlet: use AddCapability= (the generator rejects CapAdd=)

The podman quadlet generator fails with "unsupported key 'CapAdd' in group
'Container'" and skips the whole unit. The correct key is AddCapability=
(maps to podman --cap-add). Update the gluetun unit and its test seam.

tests/searxng_quadlet.bats

@@ -37,8 +37,8 @@ QDIR="$REPO_ROOT/dankclaw/searxng/quadlet"
 }
 
 @test "gluetun has NET_ADMIN and the /dev/net/tun device" {
-    grep -qE '^CapAdd=NET_ADMIN' "$QDIR/gluetun.container" \
-        || { echo "gluetun.container must set CapAdd=NET_ADMIN"; return 1; }
+    grep -qE '^AddCapability=NET_ADMIN' "$QDIR/gluetun.container" \
+        || { echo "gluetun.container must set AddCapability=NET_ADMIN"; return 1; }
     grep -qE '^Device=/dev/net/tun' "$QDIR/gluetun.container" \
         || { echo "gluetun.container must set Device=/dev/net/tun"; return 1; }
 }