chore(lint): add musttag linter and fix lint errors

Breaking change: JSON fields changed in the server API

internal/configuration/settings/vpn.go

@@ -13,10 +13,10 @@ type VPN struct {
 	// Type is the VPN type and can only be
 	// 'openvpn' or 'wireguard'. It cannot be the
 	// empty string in the internal state.
-	Type      string
-	Provider  Provider
-	OpenVPN   OpenVPN
-	Wireguard Wireguard
+	Type      string    `json:"type"`
+	Provider  Provider  `json:"provider"`
+	OpenVPN   OpenVPN   `json:"openvpn"`
+	Wireguard Wireguard `json:"wireguard"`
 }
 
 // TODO v4 remove pointer for receiver (because of Surfshark).