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/interface.go

@@ -18,6 +18,8 @@ type Stats struct {
 	Peers                  int
 	FileSizeBytes          int64
 	LiveRoutes             int
+	IPv4Routes             int
+	IPv6Routes             int
 	OldestRoute            *time.Time
 	NewestRoute            *time.Time
 	IPv4PrefixDistribution []PrefixDistribution