Serve /api/v1/stats counts from realtime in-memory counters (closes #27)

The stats endpoints recomputed every count with a COUNT(*) over each
table plus a MIN/MAX union scan on every request, which took the whole
timeout and returned 500 once the database passed a few GiB.

The row counts (ASNs, prefixes, peerings, peers, live routes) now live
in memory: seeded once at startup and adjusted on each write as routes
and records are inserted, updated and removed, so a read serves them
with no scan and no timer. A live-route upsert is an UPDATE, then an
INSERT only when no row matched, so its rows-affected count tells a new
route from a re-announced one. Oldest/newest timestamps read one row
from each end of the last_updated index, which also fixes the per-call
scan warning. The distinct-prefix histogram stays a single indexed
query.

JSON shape of /api/v1/stats and the status page is unchanged.

Model: opus-4-8

internal/database/counts.go

@@ -0,0 +1,146 @@
+package database
+
+import (
+	"context"
+	"fmt"
+	"sync"
+)
+
+// liveCounts holds the running row counts that the stats endpoints report. They
+// are seeded once at startup from the tables and then adjusted on every write,
+// so a stats read serves them from memory instead of running a COUNT(*) over
+// each table. Those scans, once the database passed a few GiB, took the whole
+// request timeout and made /api/v1/stats return 500 (issue 27).
+//
+// A single mutex guards all fields so the stats reader takes a consistent
+// snapshot at one instant and writers, which already run under the database
+// write lock, adjust the counts after their transaction commits.
+type liveCounts struct {
+	mu         sync.RWMutex
+	asns       int
+	prefixesV4 int
+	prefixesV6 int
+	peerings   int
+	peers      int
+	routesV4   int
+	routesV6   int
+}
+
+// seed sets every count to the value read from the tables at startup. It runs
+// before any writer, so it needs no coordination with the adjust methods.
+func (c *liveCounts) seed(asns, prefixesV4, prefixesV6, peerings, peers, routesV4, routesV6 int) {
+	c.mu.Lock()
+	defer c.mu.Unlock()
+
+	c.asns = asns
+	c.prefixesV4 = prefixesV4
+	c.prefixesV6 = prefixesV6
+	c.peerings = peerings
+	c.peers = peers
+	c.routesV4 = routesV4
+	c.routesV6 = routesV6
+}
+
+// addASNs adds n to the ASN count.
+func (c *liveCounts) addASNs(n int) {
+	c.mu.Lock()
+	c.asns += n
+	c.mu.Unlock()
+}
+
+// addPrefixes adds to the IPv4 and IPv6 prefix counts.
+func (c *liveCounts) addPrefixes(v4, v6 int) {
+	c.mu.Lock()
+	c.prefixesV4 += v4
+	c.prefixesV6 += v6
+	c.mu.Unlock()
+}
+
+// addPeerings adds n to the peering count.
+func (c *liveCounts) addPeerings(n int) {
+	c.mu.Lock()
+	c.peerings += n
+	c.mu.Unlock()
+}
+
+// addPeers adds n to the BGP peer count.
+func (c *liveCounts) addPeers(n int) {
+	c.mu.Lock()
+	c.peers += n
+	c.mu.Unlock()
+}
+
+// addRoutes adds to the IPv4 and IPv6 live-route counts. Deletions pass
+// negative values.
+func (c *liveCounts) addRoutes(v4, v6 int) {
+	c.mu.Lock()
+	c.routesV4 += v4
+	c.routesV6 += v6
+	c.mu.Unlock()
+}
+
+// fill copies the counts into a Stats, including the derived totals, under a
+// single read lock so the reader sees one consistent snapshot.
+func (c *liveCounts) fill(s *Stats) {
+	c.mu.RLock()
+	defer c.mu.RUnlock()
+
+	s.ASNs = c.asns
+	s.IPv4Prefixes = c.prefixesV4
+	s.IPv6Prefixes = c.prefixesV6
+	s.Prefixes = c.prefixesV4 + c.prefixesV6
+	s.Peerings = c.peerings
+	s.Peers = c.peers
+	s.IPv4Routes = c.routesV4
+	s.IPv6Routes = c.routesV6
+	s.LiveRoutes = c.routesV4 + c.routesV6
+}
+
+// countRows returns the number of rows in the named table. It is used only at
+// startup to seed the in-memory counters, so a full COUNT(*) is acceptable.
+func (d *Database) countRows(ctx context.Context, table string) (int, error) {
+	var n int
+	// table is one of a fixed set of literals below, never external input.
+	if err := d.db.QueryRowContext(ctx, "SELECT COUNT(*) FROM "+table).Scan(&n); err != nil {
+		return 0, fmt.Errorf("failed to count %s: %w", table, err)
+	}
+
+	return n, nil
+}
+
+// seedCounts reads the current row counts from the tables into the in-memory
+// counters. It runs once at startup, before the streamer begins writing.
+func (d *Database) seedCounts(ctx context.Context) error {
+	asns, err := d.countRows(ctx, "asns")
+	if err != nil {
+		return err
+	}
+	prefixesV4, err := d.countRows(ctx, "prefixes_v4")
+	if err != nil {
+		return err
+	}
+	prefixesV6, err := d.countRows(ctx, "prefixes_v6")
+	if err != nil {
+		return err
+	}
+	peerings, err := d.countRows(ctx, "peerings")
+	if err != nil {
+		return err
+	}
+	peers, err := d.countRows(ctx, "bgp_peers")
+	if err != nil {
+		return err
+	}
+	routesV4, err := d.countRows(ctx, "live_routes_v4")
+	if err != nil {
+		return err
+	}
+	routesV6, err := d.countRows(ctx, "live_routes_v6")
+	if err != nil {
+		return err
+	}
+
+	d.counts.seed(asns, prefixesV4, prefixesV6, peerings, peers, routesV4, routesV6)
+
+	return nil
+}