fix(admin): licence and plan features serialise as [], never null

Free has no features, so a nil Go slice served "features": null. The
portal's type said string[], called .length on it, and took the page down.

The guarantee lives on a named Features type with its own MarshalJSON
rather than at each of the six places a licence or plan is serialised,
because the seventh is the one that would have been forgotten. Applying it
at marshal time also fixes rows already holding null in Mongo, which a
write-side fix alone would not.

Same class, found while checking the rest: staffListInstances built its
expiring filter from a nil []string, marshalling to $in: null, which Mongo
rejects — so the quiet week when nothing is expiring is precisely when the
staff Operations page's own query would have failed.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

shared/license/plans.go

@@ -22,7 +22,9 @@ var plans = map[string]Plan{
 		Name:       "Free",
 		Deployment: DeploymentCloud, // cloud only, by construction
 		Limits:     Limits{MaxServers: 3, MaxSecretGroups: 1, MaxChannels: 1},
-		Features:   nil,
+		// Empty rather than nil: nil marshals as JSON null, and this table is
+		// the seed every plan and licence is cut from.
+		Features: []string{},
 	},
 	TierProfessional: {
 		Tier:       TierProfessional,