Serve /api/v1/stats from a cache and index-scan the route timestamps (closes #27)

Once the database passed about 4.5 GiB every stats request ran a COUNT(*)
over each table plus a MIN/MAX union scan of both route tables, took the
full timeout and returned HTTP 500, so the status page went blank.

The server now keeps the last database statistics in memory and recomputes
them at most once every 30 seconds; requests serve the cached copy and a
stale copy triggers a single background refresh, so no request runs the
scans. The route-count split is folded into the cached stats, removing the
separate per-request live-route count query.

The oldest/newest route timestamps now read one row from each end of the
last_updated index instead of scanning both tables, and select the column
directly so the driver parses it into time.Time; the old aggregate returned
an untyped string that failed to scan and logged a warning every call.

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