Add the ProtonVPN server networks and addresses to the IP lookup (closes #236)

Proton's own server list answers only a logged-in session, so the lists come from the public mirror https://github.com/tn3w/ProtonVPN-IPs, downloaded once a day: `protonvpn`, its /24 networks, and `protonvpn_entry`, its servers' IPv4 and IPv6 addresses. Both feed `flags.is_vpn` and `vpn_providers`.

Judgement call: the /24 networks are listed as well as the exact addresses, because coverage matters more than attribution, so neighbouring addresses in those networks count too.
Risk: the mirror is one person's repository; if it stops updating, downloads keep succeeding with old data.

Model: opus-5-5

README.md

@@ -431,6 +431,14 @@ The lists:
 - `pia`: VPN servers run by Private Internet Access. From
   [Private Internet Access](https://serverlist.piaservers.net/vpninfo/servers/v6),
   downloaded once a day.
+- `protonvpn`: the /24 networks that hold ProtonVPN servers. The whole /24 is
+  listed because the ProtonVPN-IPs README says traffic leaves from unpublished
+  addresses in it. From the public
+  [ProtonVPN-IPs](https://github.com/tn3w/ProtonVPN-IPs) mirror of Proton's
+  server list, downloaded once a day.
+- `protonvpn_entry`: the IPv4 and IPv6 addresses of ProtonVPN servers. From the
+  same [ProtonVPN-IPs](https://github.com/tn3w/ProtonVPN-IPs) mirror, downloaded
+  once a day.
 - `spamhaus_drop` and `spamhaus_dropv6`: IPv4 and IPv6 networks hijacked or
   leased by spammers or cyber criminals, according to Spamhaus. From
   [Spamhaus DROP](https://www.spamhaus.org/blocklists/do-not-route-or-peer/),