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.go

@@ -972,6 +972,34 @@ func (d *Database) GetAS(ctx context.Context, asn int) (*ASN, bool, error) {
 	return &asnInfo, true, nil
 }
 
+// The queries of GetLastSeenASes. The index on last_seen answers each without
+// reading the whole table.
+const (
+	newestLastSeenASQuery = `
+	SELECT asn, handle, description, first_seen, last_seen
+	FROM asns ORDER BY last_seen DESC LIMIT 1`
+	oldestLastSeenASQuery = `
+	SELECT asn, handle, description, first_seen, last_seen
+	FROM asns ORDER BY last_seen ASC LIMIT 1`
+)
+
+// GetLastSeenASes returns two ASes of the asns table, with their handles,
+// descriptions and seen times: the one last seen most recently, and the one
+// last seen longest ago. Both are nil when the table holds no AS.
+func (d *Database) GetLastSeenASes(ctx context.Context) (*ASN, *ASN, error) {
+	newest, found, err := d.readOneAS(ctx, newestLastSeenASQuery)
+	if err != nil || !found {
+		return nil, nil, err
+	}
+
+	oldest, _, err := d.readOneAS(ctx, oldestLastSeenASQuery)
+	if err != nil {
+		return nil, nil, err
+	}
+
+	return newest, oldest, nil
+}
+
 // asWHOISQuery reads the handle, description and WHOIS details of one AS, and
 // when the WHOIS details were fetched.
 const asWHOISQuery = `
@@ -1379,6 +1407,29 @@ func (d *Database) Ping(ctx context.Context) error {
 	return nil
 }
 
+// readOneAS returns the AS that query reads, and whether it read one.
+func (d *Database) readOneAS(ctx context.Context, query string) (*ASN, bool, error) {
+	var (
+		asnInfo             ASN
+		handle, description sql.NullString
+	)
+
+	err := d.db.QueryRowContext(ctx, query).Scan(&asnInfo.ASN, &handle,
+		&description, &asnInfo.FirstSeen, &asnInfo.LastSeen)
+	if errors.Is(err, sql.ErrNoRows) {
+		return nil, false, nil
+	}
+
+	if err != nil {
+		return nil, false, fmt.Errorf("failed to query AS by last seen: %w", err)
+	}
+
+	asnInfo.Handle = handle.String
+	asnInfo.Description = description.String
+
+	return &asnInfo, true, nil
+}
+
 // truncateWAL runs the TRUNCATE checkpoint of Checkpoint on a WAL file of
 // walBytes, with the writing connection's busy timeout lowered to
 // truncateBusyTimeoutMs for it, and logs how long it took. The caller holds the