feat: add NATS cache sharing and smart cache refresh

Add two complementary features to reduce remote revision cache staleness:

1. Smart local cache: When current system revision matches cached remote
   revision, force an immediate cache refresh to check for newer revisions.

2. NATS integration: Share cache updates across hosts via NATS pub/sub.
   Hosts publish revision updates when they fetch new data, and subscribe
   to receive updates from other hosts. Features include:
   - Auto-reconnect with infinite retries
   - Graceful fallback when NATS unavailable
   - Filtering by flake URL and hostname

New CLI flags:
  --flake.nats.enable
  --flake.nats.url
  --flake.nats.subject
  --flake.nats.credentials-file

New NixOS module options under services.prometheus.exporters.nixos.flake.nats

Bumps version to 0.3.0.

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

main.go

@@ -15,7 +15,7 @@ import (
 	"github.com/prometheus/client_golang/prometheus/promhttp"
 )
 
-const version = "0.2.3"
+const version = "0.3.0"
 
 func main() {
 	cfg, err := config.Parse()
@@ -30,10 +30,23 @@ func main() {
 	slog.Info("Registered generation collector")
 
 	// Register flake collector if enabled
+	var flakeCollector *collector.FlakeCollector
 	if cfg.FlakeCollector {
-		flakeCollector := collector.NewFlakeCollector(cfg.FlakeURL, cfg.FlakeCheckInterval)
+		var err error
+		flakeCollector, err = collector.NewFlakeCollectorWithNATS(collector.FlakeCollectorConfig{
+			FlakeURL:            cfg.FlakeURL,
+			CheckInterval:       cfg.FlakeCheckInterval,
+			NATSEnabled:         cfg.FlakeNATSEnable,
+			NATSURL:             cfg.FlakeNATSURL,
+			NATSSubject:         cfg.FlakeNATSSubject,
+			NATSCredentialsFile: cfg.FlakeNATSCredentialsFile,
+		})
+		if err != nil {
+			slog.Error("Failed to create flake collector", "error", err)
+			os.Exit(1)
+		}
 		prometheus.MustRegister(flakeCollector)
-		slog.Info("Registered flake collector", "url", cfg.FlakeURL, "check_interval", cfg.FlakeCheckInterval)
+		slog.Info("Registered flake collector", "url", cfg.FlakeURL, "check_interval", cfg.FlakeCheckInterval, "nats_enabled", cfg.FlakeNATSEnable)
 	}
 
 	mux := http.NewServeMux()
@@ -71,6 +84,13 @@ func main() {
 	<-ctx.Done()
 	slog.Info("Shutting down server")
 
+	// Close flake collector (NATS connection)
+	if flakeCollector != nil {
+		if err := flakeCollector.Close(); err != nil {
+			slog.Error("Failed to close flake collector", "error", err)
+		}
+	}
+
 	shutdownCtx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
 	defer cancel()