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

Every IP lookup result gains `geo`. One request to geojs carries all uncached addresses of a lookup, within 1 second. On a timeout, an error or an answer with no location, the lookup reads DB-IP City Lite, downloaded monthly into the state directory. Geojs answers stay in memory, at most 250,000, and in `cache.db` for 7 days. The home page footer credits DB-IP.

Judgement call: a reserved address goes to neither source, and its `geo` is null.
Judgement call: `updated` for a DB-IP answer is when the copy was downloaded.
Deviation: the cache is SQLite, by sneak's words, not the JSON file the standing rule asks for.

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}}">