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/templates/index.html

@@ -544,6 +544,8 @@
         <span>{{appLicense}}</span>
         <span class="separator">|</span>
         <span><a href="{{appGitCommitURL}}">{{appGitRevision}}</a></span>
+        <span class="separator">|</span>
+        <span>IP Geolocation by <a href="https://db-ip.com">DB-IP</a></span>
     </footer>
 
     <script nonce="{{.CSPNonce}}">