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}}">

internal/templates/templates_test.go

@@ -67,6 +67,23 @@ func TestFooterLicense(t *testing.T) {
 	}
 }
 
+// TestHomePageCreditsDBIP expects the home page's footer to carry the credit
+// to DB-IP.
+func TestHomePageCreditsDBIP(t *testing.T) {
+	t.Parallel()
+
+	const attribution = `IP Geolocation by <a href="https://db-ip.com">DB-IP</a>`
+
+	_, footer, found := strings.Cut(renderPage(t, templates.IndexTemplate()), "<footer")
+	if !found {
+		t.Fatal("the page has no footer")
+	}
+
+	if !strings.Contains(footer, attribution) {
+		t.Errorf("footer does not credit DB-IP: %q", footer)
+	}
+}
+
 // TestFreshnessIndicator renders the home and status pages and expects each to
 // have, right after the status element, the element where the page's script
 // shows its spinner or "stale".