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/routewatch/app_integration_test.go

@@ -221,6 +221,11 @@ func (m *mockStore) GetPrefixDetails(prefix string) ([]database.LiveRoute, error
 	return []database.LiveRoute{}, nil
 }
 
+func (m *mockStore) GetRandomPrefixesByLength(maskLength, ipVersion, limit int) ([]database.LiveRoute, error) {
+	// Return empty routes for now
+	return []database.LiveRoute{}, nil
+}
+
 // UpsertLiveRouteBatch mock implementation
 func (m *mockStore) UpsertLiveRouteBatch(routes []*database.LiveRoute) error {
 	m.mu.Lock()