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

internal/cachedb/schema.sql

@@ -6,3 +6,19 @@ CREATE TABLE IF NOT EXISTS hostnames (
     names TEXT NOT NULL,
     expires_at INTEGER NOT NULL
 );
+
+-- geolocations holds the answers of geojs. A part geojs does not know is NULL.
+-- fetched_at is in seconds since 1970.
+CREATE TABLE IF NOT EXISTS geolocations (
+    address TEXT PRIMARY KEY,
+    country_code TEXT,
+    country TEXT,
+    continent_code TEXT,
+    region TEXT,
+    city TEXT,
+    latitude REAL,
+    longitude REAL,
+    accuracy_radius_km INTEGER,
+    time_zone TEXT,
+    fetched_at INTEGER NOT NULL
+);