Add best-effort geolocation to the IP lookup from geojs and DB-IP City Lite (closes #239)

Each result gains `geo`. One request to geojs carries every address, 1 second in all. On a timeout or error the lookup reads DB-IP City Lite, downloaded monthly into the state directory. geojs answers stay in memory (250,000, least recently used dropped) and in a new `cache.db` table for 7 days.

Judgement call: a reserved address goes to neither source, and its `geo` is null.
Judgement call: an answer from geojs with every part unknown is still an answer.
Judgement call: `updated` for DB-IP is when the copy was downloaded.
Deviation: the cache is SQLite, by sneak's words.

Model: opus-5-5

internal/server/server.go

@@ -24,6 +24,7 @@ import (
 
 	"git.eeqj.de/sneak/routewatch/internal/config"
 	"git.eeqj.de/sneak/routewatch/internal/database"
+	"git.eeqj.de/sneak/routewatch/internal/geo"
 	"git.eeqj.de/sneak/routewatch/internal/iplist"
 	"git.eeqj.de/sneak/routewatch/internal/logger"
 	"git.eeqj.de/sneak/routewatch/internal/routetable"
@@ -63,6 +64,12 @@ type PTRResolver interface {
 	LookupAddr(ctx context.Context, addr string) ([]string, error)
 }
 
+// GeoLocator finds where addresses are, as *geo.Locator does. It returns one
+// entry for each address, nil when no source answers.
+type GeoLocator interface {
+	Lookup(ctx context.Context, addrs []netip.Addr) []*geo.Location
+}
+
 // Server provides HTTP endpoints for status monitoring
 type Server struct {
 	router     *chi.Mux
@@ -86,6 +93,10 @@ type Server struct {
 	// ptrResolver looks up the IP lookup's PTR names. SetPTRResolver sets it.
 	ptrResolver PTRResolver
 
+	// geoLocator finds where the IP lookup's addresses are. SetGeoLocator
+	// sets it.
+	geoLocator GeoLocator
+
 	// cards is the copy of the home page's cards that the pages serve, nil
 	// until the server first works them out
 	cards atomic.Pointer[homeCards]
@@ -209,6 +220,12 @@ func (s *Server) SetPTRResolver(resolver PTRResolver) {
 	s.ptrResolver = resolver
 }
 
+// SetGeoLocator sets what finds where the IP lookup's addresses are. Call it
+// before Start: the handlers read it without a lock.
+func (s *Server) SetGeoLocator(locator GeoLocator) {
+	s.geoLocator = locator
+}
+
 // SetASNFetcher sets the ASN WHOIS fetcher for on-demand lookups. Call it
 // before Start: the handlers read the fetcher without a lock.
 func (s *Server) SetASNFetcher(fetcher ASNFetcher) {