fix: production engine/module shutdown — close DB connections and stop goroutines properly

- Add Stop() to FeatureFlagModule to close its SQLite store on shutdown
  (was leaking the DB connection since the module never implemented Stoppable)
- Add 30-second timeout to multi-workflow shutdown context to prevent
  indefinite hangs if in-flight requests or modules fail to stop
- Add 30-second timeout to RunUntilSignal shutdown for the same reason

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

Author: intel352

cmd/server/main.go

@@ -1671,8 +1671,10 @@ func runMultiWorkflow(logger *slog.Logger) error {
 	}
 	cancel()
 
-	// Graceful shutdown
-	shutdownCtx := context.Background()
+	// Graceful shutdown with a 30-second timeout to avoid blocking forever
+	// if in-flight requests or engine modules hang.
+	shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer shutdownCancel()
 	if err := mgr.StopAll(shutdownCtx); err != nil {
 		logger.Error("Engine manager shutdown error", "error", err)
 	}

engine_builder.go

@@ -7,6 +7,7 @@ import (
 	"os"
 	"os/signal"
 	"syscall"
+	"time"
 
 	"github.com/GoCodeAlone/modular"
 	"github.com/GoCodeAlone/workflow/config"
@@ -266,7 +267,9 @@ func (b *EngineBuilder) RunUntilSignal(cfg *config.WorkflowConfig) error {
 	<-sigCh
 
 	cancel()
-	if err := engine.Stop(context.Background()); err != nil {
+	shutdownCtx, shutdownCancel := context.WithTimeout(context.Background(), 30*time.Second)
+	defer shutdownCancel()
+	if err := engine.Stop(shutdownCtx); err != nil {
 		return fmt.Errorf("shutdown error: %w", err)
 	}
 	return nil

module/feature_flag_module.go

@@ -1,6 +1,7 @@
 package module
 
 import (
+	"context"
 	"fmt"
 	"log/slog"
 	"os"
@@ -126,6 +127,14 @@ func (m *FeatureFlagModule) Store() *generic.Store {
 	return m.store
 }
 
+// Stop closes the underlying store's database connection during shutdown.
+func (m *FeatureFlagModule) Stop(_ context.Context) error {
+	if m.store != nil {
+		return m.store.Close()
+	}
+	return nil
+}
+
 // SSEEnabled returns whether SSE streaming is enabled for this module.
 func (m *FeatureFlagModule) SSEEnabled() bool {
 	return m.config.SSEEnabled