Report which public blocklists list an address in the IP lookup (closes #158)

A new package, internal/blocklist, downloads seven public lists on a
schedule: DShield, Feodo Tracker, FireHOL abusers and level 3, Spamhaus
DROP for IPv4 and IPv6, and the Tor exit list. It keeps the last good
copy of each in memory and on disk. A download sends back the
Last-Modified and ETag the publisher gave for the copy, also kept on
disk.

Every IP lookup result carries blocklists. An address with no live route
now gets a result too, without route fields, so the lookup no longer
answers 404. A successful answer carries Cache-Control, private for the
caller's own address. The address sets come from go4.org/netipx.

Model: opus-5-5

internal/server/server.go

@@ -15,6 +15,7 @@ import (
 	"strconv"
 	"time"
 
+	"git.eeqj.de/sneak/routewatch/internal/blocklist"
 	"git.eeqj.de/sneak/routewatch/internal/config"
 	"git.eeqj.de/sneak/routewatch/internal/database"
 	"git.eeqj.de/sneak/routewatch/internal/logger"
@@ -36,6 +37,13 @@ type ASNFetcher interface {
 	GetStats() ASNFetcherStats
 }
 
+// Blocklists tells which public blocklists list an address, from copies it
+// keeps in memory, as *blocklist.Blocklists does.
+type Blocklists interface {
+	Lookup(addr netip.Addr) []blocklist.Listing
+	ShortestInterval() time.Duration
+}
+
 // dnsResolver looks up the addresses of host names and the names of addresses
 // in DNS, as *net.Resolver does.
 type dnsResolver interface {
@@ -52,6 +60,7 @@ type Server struct {
 	port       int
 	srv        *http.Server
 	asnFetcher ASNFetcher
+	blocklists Blocklists
 
 	// trustedProxies are the reverse proxies whose X-Forwarded-For header
 	// RealIPMiddleware believes
@@ -148,3 +157,9 @@ func (s *Server) Stop(ctx context.Context) error {
 func (s *Server) SetASNFetcher(fetcher ASNFetcher) {
 	s.asnFetcher = fetcher
 }
+
+// SetBlocklists sets the blocklists the IP lookups report. Call it before
+// Start: the handlers read the blocklists without a lock.
+func (s *Server) SetBlocklists(blocklists Blocklists) {
+	s.blocklists = blocklists
+}