fix: surface vuln_scanning across licence, staff and pricing UI

The catalogue row alone was not enough; the feature was invisible in
three places and mislabelled in a fourth.

PlanConfigurator rendered every key that was not "console" as "Single
sign-on", so the staff checkbox granting vulnerability scanning was
labelled single sign-on. Feature wording was duplicated between the staff
configurator and the purchase form and the copies had drifted, so it now
lives in adminsite/lib/features.ts and both read from it.

The customer licence panel showed raw keys; it now labels them.

Pricing gains a comparison row. The add-on block with a monthly price is
deliberately NOT added: that is a pricing decision, and the Paddle price
IDs for the new catalogue rows have to be pasted in before it can be sold
anyway.

admin/internal/models/catalogue.go

@@ -67,7 +67,8 @@ func (r CatalogueRow) Priced(env string) bool {
 	return false
 }
 
-// SeedCatalogue inserts the sixteen rows the four PAID plans need.
+// SeedCatalogue inserts the twenty rows the four PAID plans need: a base, a
+// server limit, and one row per feature key.
 //
 // The two Free plans get no rows at all, and that absence is what keeps Free
 // outside Paddle: with nothing to price, no checkout can be built for it. Do not