Implement routing table snapshotter with automatic loading on startup

- Create snapshotter package with periodic (10 min) and on-demand snapshots
- Add JSON serialization with gzip compression and atomic file writes
- Update routing table to track AddedAt time for each route
- Load snapshots on startup, filtering out stale routes (>30 minutes old)
- Add ROUTEWATCH_DISABLE_SNAPSHOTTER env var for tests
- Use OS-appropriate state directories (macOS: ~/Library/Application Support, Linux: /var/lib or XDG_STATE_HOME)

internal/routewatch/app_integration_test.go

@@ -156,6 +156,9 @@ func (m *mockStore) GetStats() (database.Stats, error) {
 }
 
 func TestRouteWatchLiveFeed(t *testing.T) {
+	// Disable snapshotter for tests
+	t.Setenv("ROUTEWATCH_DISABLE_SNAPSHOTTER", "1")
+
 	// Create mock database
 	mockDB := newMockStore()
 	defer mockDB.Close()
@@ -169,7 +172,7 @@ func TestRouteWatchLiveFeed(t *testing.T) {
 	s := streamer.New(logger, metricsTracker)
 
 	// Create routing table
-	rt := routingtable.New()
+	rt := routingtable.New(logger)
 
 	// Create server
 	srv := server.New(mockDB, rt, s, logger)