fix: auto-enable cluster bootstrap, add init-db.sql to postgres volumes

- Auto-enable cluster seeding when BOOTSTRAP_CLUSTERS is set
  (no longer requires separate BOOTSTRAP_ENABLE_CLUSTERS=true)
- Add BOOTSTRAP_ENABLE_CLUSTERS to README .env template
- Mount init-db.sql in postgres service volumes

backend/internal/bootstrap/config.go

@@ -138,7 +138,9 @@ func loadBootstrapConfigFromEnv() (*BootstrapConfig, bool) {
 		})
 	}
 
-	if parseBoolEnv("BOOTSTRAP_ENABLE_CLUSTERS", false) {
+	enableClusters := parseBoolEnv("BOOTSTRAP_ENABLE_CLUSTERS", false) ||
+		os.Getenv("BOOTSTRAP_CLUSTERS") != ""
+	if enableClusters {
 		for _, clusterName := range discoverBootstrapClusters() {
 			prefix := "BOOTSTRAP_CLUSTER_" + normalizeEnvName(clusterName) + "_"
 			host := os.Getenv(prefix + "HOST")