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 and address space 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/database/database_internal_test.go

@@ -1003,6 +1003,62 @@ func TestGetASReadsTheRecordOrTheBuiltInList(t *testing.T) {
 	}
 }
 
+// TestGetLastSeenASes stores three ASes, each last seen at another time, and
+// expects the one last seen most recently and the one last seen longest ago.
+// An empty asns table gives neither.
+func TestGetLastSeenASes(t *testing.T) {
+	t.Parallel()
+
+	db, err := New(&config.Config{StateDir: t.TempDir()}, logger.New())
+	if err != nil {
+		t.Fatalf("failed to create database: %v", err)
+	}
+	defer func() { _ = db.Close() }()
+
+	newest, oldest, err := db.GetLastSeenASes(t.Context())
+	if err != nil || newest != nil || oldest != nil {
+		t.Fatalf("GetLastSeenASes() on an empty table = %+v, %+v, %v; want "+
+			"neither", newest, oldest, err)
+	}
+
+	const (
+		newestASN = 13335
+		middleASN = 15169
+		oldestASN = 3
+	)
+
+	start := time.Date(2026, 1, 2, 3, 4, 5, 0, time.UTC)
+
+	err = db.GetOrCreateASNBatch(map[int]SeenTimes{
+		middleASN: {First: start, Last: start.Add(time.Hour)},
+		newestASN: {First: start, Last: start.Add(2 * time.Hour)},
+		oldestASN: {First: start, Last: start.Add(time.Minute)},
+	})
+	if err != nil {
+		t.Fatalf("GetOrCreateASNBatch: %v", err)
+	}
+
+	newest, oldest, err = db.GetLastSeenASes(t.Context())
+	if err != nil {
+		t.Fatalf("GetLastSeenASes: %v", err)
+	}
+
+	listed, _ := asinfo.Get(newestASN)
+
+	if newest == nil || newest.ASN != newestASN ||
+		!newest.LastSeen.Equal(start.Add(2*time.Hour)) ||
+		newest.Description != listed.Description {
+		t.Errorf("newest = %+v, want AS%d (%s) last seen %v", newest, newestASN,
+			listed.Description, start.Add(2*time.Hour))
+	}
+
+	if oldest == nil || oldest.ASN != oldestASN ||
+		!oldest.LastSeen.Equal(start.Add(time.Minute)) {
+		t.Errorf("oldest = %+v, want AS%d last seen %v", oldest, oldestASN,
+			start.Add(time.Minute))
+	}
+}
+
 // TestGetASWHOISReadsTheStoredDetails stores an AS with WHOIS details and asks
 // GetASWHOIS for it and for an AS the asns table does not hold. The first gets
 // its stored handle, description and details, and the time they were fetched.