Add the iCloud Private Relay list to the IP lookup, with is_relay (closes #237)

Adds Apple's iCloud Private Relay egress list as `icloud_private_relay`, downloaded once a day, and `flags.is_relay`, true when an address is on it. Loaded, the list adds about 0.3 MiB to the Go heap; parsing it allocates about 94 MiB, freed by the next garbage collection.

Judgement call: a line with no comma fails the parse, so a plain network list served at Apple's URL cannot replace a good copy.

Model: opus-5-5

internal/server/handlers.go

@@ -797,6 +797,9 @@ type IPLookupFlags struct {
 	// IsVPN says that the address is on a list of VPN servers.
 	IsVPN bool `json:"is_vpn"`
 
+	// IsRelay says that the address is on the iCloud Private Relay list.
+	IsRelay bool `json:"is_relay"`
+
 	// IsBogon says that the address is reserved, as package bogon tells.
 	IsBogon bool `json:"is_bogon"`
 }
@@ -1025,6 +1028,9 @@ func ipLookupFlags(ip string, listings []iplist.Listing) IPLookupFlags {
 		IsVPN: slices.ContainsFunc(listings, func(l iplist.Listing) bool {
 			return l.VPN
 		}),
+		IsRelay: slices.ContainsFunc(listings, func(l iplist.Listing) bool {
+			return l.Name == iplist.ICloudPrivateRelayList
+		}),
 		IsBogon: isBogon(ip),
 	}
 }