Add oldest and newest route timestamps to status page

Display oldest and newest route timestamps in the Routing Table card
on the /status page. Timestamps are shown as relative times (e.g.,
"5m ago", "2h 30m ago").

Changes:
- Add OldestRoute and NewestRoute fields to database.Stats
- Query MIN/MAX of last_updated across live_routes_v4 and v6 tables
- Include timestamps in both /status.json and /api/v1/stats responses
- Add formatRelativeTime JavaScript function for display

internal/database/interface.go

@@ -18,6 +18,8 @@ type Stats struct {
 	Peers                  int
 	FileSizeBytes          int64
 	LiveRoutes             int
+	OldestRoute            *time.Time
+	NewestRoute            *time.Time
 	IPv4PrefixDistribution []PrefixDistribution
 	IPv6PrefixDistribution []PrefixDistribution
 }