Show figures of the routing table on the home page (closes #154)

The home page gets cards for the newest and oldest kept route, the newest
and oldest updated AS, the AS with the most prefixes, the ASes with a
single prefix and fifteen well-known networks. Two more cards count the
bogon announcements and the prefixes with several origin ASes. They link
to the new pages /bogons and /multiple-origins, which list up to 1,000
entries each.

The new package internal/bogon tells reserved prefixes and AS numbers,
from the IANA special-purpose registries. Table.Overview walks the table
and lets the read lock go every 10,000 prefixes. The server works the
cards out once a minute and serves that copy. A new index on
asns(last_seen) finds the newest and oldest updated AS.

Model: opus-5-5

internal/server/server.go

@@ -1,5 +1,7 @@
 // Package server serves RouteWatch's web pages and JSON API:
 //   - the home page, the status figures and the health check
+//   - the pages that list the bogon announcements and the prefixes announced
+//     by more than one origin AS
 //   - the AS, prefix and prefix length pages, and the JSON answers behind them
 //   - the IP lookups
 //   - the AS name search
@@ -17,6 +19,7 @@ import (
 	"net/http"
 	"net/netip"
 	"strconv"
+	"sync/atomic"
 	"time"
 
 	"git.eeqj.de/sneak/routewatch/internal/blocklist"
@@ -75,6 +78,10 @@ type Server struct {
 	// resolver makes the IP lookup's DNS lookups: net.DefaultResolver, or a
 	// fake in tests
 	resolver dnsResolver
+
+	// cards is the copy of the home page's cards that the pages serve, nil
+	// until the server first works them out
+	cards atomic.Pointer[homeCards]
 }
 
 // New creates a new HTTP server that reads routes from table, the routing
@@ -103,7 +110,8 @@ func New(
 
 // Start listens on the configured port and serves HTTP in the background.
 // It returns an error if it cannot listen, for example because the port is
-// already in use. ctx bounds only opening the port, not serving.
+// already in use. ctx bounds opening the port, not serving. Once listening,
+// the server works out the home page's cards every minute until ctx ends.
 func (s *Server) Start(ctx context.Context) error {
 	const (
 		network           = "tcp"
@@ -140,6 +148,10 @@ func (s *Server) Start(ctx context.Context) error {
 		}
 	}()
 
+	go func() {
+		s.keepHomeCardsFresh(ctx, homeCardsRefreshInterval)
+	}()
+
 	return nil
 }