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/server/routes.go

@@ -28,6 +28,7 @@ func (s *Server) setupRoutes() {
 	// AS and prefix detail pages
 	r.Get("/as/{asn}", s.handleASDetail())
 	r.Get("/prefix/{prefix}", s.handlePrefixDetail())
+	r.Get("/prefixlength/{length}", s.handlePrefixLength())
 	r.Get("/ip/{ip}", s.handleIPRedirect())
 
 	// API routes