Add the public VPN server lists, with is_vpn and vpn_providers (closes #235)

The IP lookup now checks Mullvad, NordVPN, Private Internet Access, IVPN and X4BNet's community list, each downloaded once a day. Each result has `flags.is_vpn` and `vpn_providers`.

Judgement call: X4BNet publishes IPv4 and IPv6 as two files, so they are two lists, `x4bnet` and `x4bnet_v6`.
Judgement call: Private Internet Access's signature is not read or checked.
Rule suppressed: `tagliatelle` on the structs holding the publishers' field names.

Model: opus-5-5

internal/server/handlers.go

@@ -781,6 +781,10 @@ type IPLookupResult struct {
 
 	Flags IPLookupFlags `json:"flags"`
 
+	// VPNProviders names the VPN lists the address is on. It is empty, never
+	// nil, when there are none.
+	VPNProviders []string `json:"vpn_providers"`
+
 	// IPLists has one entry for each IP list the address is on.
 	IPLists []iplist.Listing `json:"ip_lists"`
 }
@@ -793,6 +797,9 @@ type IPLookupFlags struct {
 	// IsTor says that the address is on the Tor exit list.
 	IsTor bool `json:"is_tor"`
 
+	// IsVPN says that the address is on a list of VPN servers.
+	IsVPN bool `json:"is_vpn"`
+
 	// IsBogon says that the address is reserved, as package bogon tells.
 	IsBogon bool `json:"is_bogon"`
 }
@@ -977,14 +984,30 @@ func (s *Server) ipLookupResult(ip string, ptrs []string, info IPInfo) IPLookupR
 	listings := s.ipListListings(ip)
 
 	return IPLookupResult{
-		IP:      ip,
-		PTR:     ptrs,
-		IPInfo:  info,
-		Flags:   ipLookupFlags(ip, listings),
-		IPLists: listings,
+		IP:           ip,
+		PTR:          ptrs,
+		IPInfo:       info,
+		Flags:        ipLookupFlags(ip, listings),
+		VPNProviders: vpnProviders(listings),
+		IPLists:      listings,
 	}
 }
 
+// vpnProviders returns the names of the VPN lists among listings. It returns
+// an empty list, never nil, so that an answer's vpn_providers is [] rather
+// than null.
+func vpnProviders(listings []iplist.Listing) []string {
+	names := []string{}
+
+	for _, listing := range listings {
+		if listing.VPN {
+			names = append(names, listing.Name)
+		}
+	}
+
+	return names
+}
+
 // ipListListings returns the IP lists that contain ip. It returns an empty
 // list, never nil, so that an answer's ip_lists is [] rather than null.
 func (s *Server) ipListListings(ip string) []iplist.Listing {
@@ -1002,6 +1025,9 @@ func ipLookupFlags(ip string, listings []iplist.Listing) IPLookupFlags {
 		IsTor: slices.ContainsFunc(listings, func(l iplist.Listing) bool {
 			return l.Name == iplist.TorExitsList
 		}),
+		IsVPN: slices.ContainsFunc(listings, func(l iplist.Listing) bool {
+			return l.VPN
+		}),
 		IsBogon: isBogon(ip),
 	}
 }