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, an error or an answer with no country, city or coordinates, 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: `updated` for DB-IP is when the copy was downloaded.
Deviation: the cache is SQLite, by sneak's words.

Model: opus-5-5

go.mod

@@ -8,8 +8,11 @@ require (
 	github.com/go-chi/chi/v5 v5.2.2
 	github.com/google/btree v1.1.3
 	github.com/google/uuid v1.6.0
+	github.com/hashicorp/golang-lru/v2 v2.0.7
 	github.com/mattn/go-sqlite3 v1.14.29
+	github.com/maxmind/mmdbwriter v1.2.0
 	github.com/miekg/dns v1.1.73
+	github.com/oschwald/maxminddb-golang/v2 v2.9.0
 	github.com/rcrowley/go-metrics v0.0.0-20250401214520-65e299d6c5c9
 	go.uber.org/fx v1.24.0
 	go4.org/netipx v0.0.0-20260823151212-3075585bcbeb
@@ -22,5 +25,5 @@ require (
 	go.uber.org/multierr v1.10.0 // indirect
 	go.uber.org/zap v1.26.0 // indirect
 	golang.org/x/net v0.57.0 // indirect
-	golang.org/x/sys v0.47.0 // indirect
+	golang.org/x/sys v0.49.0 // indirect
 )