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/routetable/overview_internal_test.go

@@ -0,0 +1,74 @@
+package routetable
+
+import (
+	"net/netip"
+	"testing"
+	"time"
+)
+
+// TestWalkPrefixesVisitsEveryPrefix walks a table with more prefixes than two
+// of the walk's batches, while another goroutine keeps adding and removing
+// other prefixes of the same mask length. The walk lets the lock go between
+// batches, so those updates get in while it runs. Every prefix that was in
+// the table all along must be visited once.
+func TestWalkPrefixesVisitsEveryPrefix(t *testing.T) {
+	t.Parallel()
+
+	const (
+		asn  = 1000
+		peer = "192.0.2.1"
+		// prefixCount is more prefixes than two of the walk's batches.
+		prefixCount = 2*walkBatch + 1
+	)
+
+	routeTime := time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC)
+	table := New(Params{})
+	prefixes := make([]netip.Prefix, 0, prefixCount)
+	addr := netip.MustParseAddr("1.0.0.0")
+
+	for range prefixCount {
+		prefix := netip.PrefixFrom(addr, addr.BitLen())
+		table.Announce(prefix, asn, Route{Peer: peer, Time: routeTime})
+		prefixes = append(prefixes, prefix)
+		addr = addr.Next()
+	}
+
+	stop := make(chan struct{})
+	updated := make(chan struct{})
+
+	go func() {
+		defer close(updated)
+
+		other := netip.MustParseAddr("2.0.0.0")
+
+		for {
+			select {
+			case <-stop:
+				return
+			default:
+			}
+
+			prefix := netip.PrefixFrom(other, other.BitLen())
+			table.Announce(prefix, asn, Route{Peer: peer, Time: routeTime})
+			table.Withdraw(prefix, peer)
+
+			other = other.Next()
+		}
+	}()
+
+	visits := make(map[netip.Prefix]int)
+
+	table.walkPrefixes(func(prefix netip.Prefix, _ []*origin) {
+		visits[prefix]++
+	})
+
+	close(stop)
+	<-updated
+
+	for _, prefix := range prefixes {
+		if visits[prefix] != 1 {
+			t.Fatalf("the walk visited %s %d times, want once", prefix,
+				visits[prefix])
+		}
+	}
+}