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

README.md

@@ -552,7 +552,7 @@ file.
 How RouteWatch keeps the answers of geojs:
 
 - It holds at most 250,000 addresses in memory, and drops the least recently
-  used first. That takes about 75 MB of the Go heap.
+  used first. That takes about 100 MB of the Go heap.
 - It writes new answers every 5 seconds to `cache.db` in the state directory.
 - It loads the newest 250,000 answers from `cache.db` at startup.
 - An address not in memory is read from `cache.db` before geojs is asked.