Fix prefix distribution bug and add prefix length pages

- Fix GetPrefixDistribution to count unique prefixes using COUNT(DISTINCT prefix) instead of COUNT(*)
- Add /prefixlength/<length> route showing random sample of 500 prefixes
- Make prefix counts on status page clickable links to prefix length pages
- Add GetRandomPrefixesByLength database method
- Create prefix_length.html template with sortable table
- Show prefix age and origin AS with descriptions

internal/templates/status.html

@@ -231,7 +231,7 @@
                 metric.className = 'metric';
                 metric.innerHTML = `
                     <span class="metric-label">/${item.mask_length}</span>
-                    <span class="metric-value">${formatNumber(item.count)}</span>
+                    <a href="/prefixlength/${item.mask_length}" class="metric-value" style="text-decoration: none; color: inherit;">${formatNumber(item.count)}</a>
                 `;
                 container.appendChild(metric);
             });