refactor: implement unified binary architecture

🏗️ **Major Architecture Refactoring: Separate CLI + Server → Unified Binary**

**Key Changes:**
✅ **Unified Binary**: Single 'insertr' binary with subcommands (enhance, serve)
✅ **Preserved Database Architecture**: Maintained sophisticated sqlc multi-DB setup
✅ **Smart Configuration**: Viper + YAML config with CLI flag precedence
✅ **Updated Build System**: Unified justfile, Air, and npm scripts

**Command Structure:**
- `insertr enhance [input-dir]` - Build-time content injection
- `insertr serve` - HTTP API server (dev + production modes)
- `insertr --config insertr.yaml` - YAML configuration support

**Architecture Benefits:**
- **Shared Database Layer**: Single source of truth for content models
- **Flexible Workflows**: Local DB for dev, remote API for production
- **Simple Deployment**: One binary for all use cases
- **Better UX**: Consistent configuration across build and runtime

**Preserved Features:**
- Multi-database support (SQLite + PostgreSQL)
- sqlc code generation and type safety
- Version control system with rollback
- Professional API endpoints
- Content enhancement pipeline

**Development Workflow:**
- `just dev` - Full-stack development (API server + demo site)
- `just serve` - API server only
- `just enhance` - Build-time content injection
- `air` - Hot reload unified binary

**Migration:** Consolidated insertr-cli/ and insertr-server/ → unified root structure

cmd/serve.go

@@ -0,0 +1,134 @@
+package cmd
+
+import (
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+	"os/signal"
+	"syscall"
+
+	"github.com/gorilla/mux"
+	"github.com/spf13/cobra"
+	"github.com/spf13/viper"
+
+	"github.com/insertr/insertr/internal/api"
+	"github.com/insertr/insertr/internal/db"
+)
+
+var serveCmd = &cobra.Command{
+	Use:   "serve",
+	Short: "Start the content API server",
+	Long: `Start the HTTP API server that provides content storage and retrieval.
+Supports both development and production modes with SQLite or PostgreSQL databases.`,
+	Run: runServe,
+}
+
+var (
+	port    int
+	devMode bool
+)
+
+func init() {
+	serveCmd.Flags().IntVarP(&port, "port", "p", 8080, "Server port")
+	serveCmd.Flags().BoolVar(&devMode, "dev-mode", false, "Enable development mode features")
+
+	// Bind flags to viper
+	viper.BindPFlag("server.port", serveCmd.Flags().Lookup("port"))
+	viper.BindPFlag("server.dev_mode", serveCmd.Flags().Lookup("dev-mode"))
+}
+
+func runServe(cmd *cobra.Command, args []string) {
+	// Get configuration values
+	port := viper.GetInt("server.port")
+	dbPath := viper.GetString("database.path")
+	devMode := viper.GetBool("server.dev_mode")
+
+	// Initialize database
+	database, err := db.NewDatabase(dbPath)
+	if err != nil {
+		log.Fatalf("Failed to initialize database: %v", err)
+	}
+	defer database.Close()
+
+	// Initialize handlers
+	contentHandler := api.NewContentHandler(database)
+
+	// Setup router
+	router := mux.NewRouter()
+
+	// Add middleware
+	router.Use(api.CORSMiddleware)
+	router.Use(api.LoggingMiddleware)
+	router.Use(api.ContentTypeMiddleware)
+
+	// Health check endpoint
+	router.HandleFunc("/health", api.HealthMiddleware())
+
+	// API routes
+	apiRouter := router.PathPrefix("/api/content").Subrouter()
+
+	// Content endpoints matching the expected API contract
+	apiRouter.HandleFunc("/bulk", contentHandler.GetBulkContent).Methods("GET")
+	apiRouter.HandleFunc("/{id}", contentHandler.GetContent).Methods("GET")
+	apiRouter.HandleFunc("/{id}", contentHandler.UpdateContent).Methods("PUT")
+	apiRouter.HandleFunc("", contentHandler.GetAllContent).Methods("GET")
+	apiRouter.HandleFunc("", contentHandler.CreateContent).Methods("POST")
+
+	// Version control endpoints
+	apiRouter.HandleFunc("/{id}/versions", contentHandler.GetContentVersions).Methods("GET")
+	apiRouter.HandleFunc("/{id}/rollback", contentHandler.RollbackContent).Methods("POST")
+
+	// Handle CORS preflight requests explicitly
+	apiRouter.HandleFunc("/{id}", api.CORSPreflightHandler).Methods("OPTIONS")
+	apiRouter.HandleFunc("", api.CORSPreflightHandler).Methods("OPTIONS")
+	apiRouter.HandleFunc("/bulk", api.CORSPreflightHandler).Methods("OPTIONS")
+	apiRouter.HandleFunc("/{id}/versions", api.CORSPreflightHandler).Methods("OPTIONS")
+	apiRouter.HandleFunc("/{id}/rollback", api.CORSPreflightHandler).Methods("OPTIONS")
+
+	// Start server
+	addr := fmt.Sprintf(":%d", port)
+	mode := "production"
+	if devMode {
+		mode = "development"
+	}
+
+	fmt.Printf("🚀 Insertr Content Server starting (%s mode)...\n", mode)
+	fmt.Printf("📁 Database: %s\n", dbPath)
+	fmt.Printf("🌐 Server running at: http://localhost%s\n", addr)
+	fmt.Printf("💚 Health check: http://localhost%s/health\n", addr)
+	fmt.Printf("📊 API endpoints:\n")
+	fmt.Printf("   GET    /api/content?site_id={site}\n")
+	fmt.Printf("   GET    /api/content/{id}?site_id={site}\n")
+	fmt.Printf("   GET    /api/content/bulk?site_id={site}&ids[]={id1}&ids[]={id2}\n")
+	fmt.Printf("   POST   /api/content\n")
+	fmt.Printf("   PUT    /api/content/{id}\n")
+	fmt.Printf("   GET    /api/content/{id}/versions?site_id={site}\n")
+	fmt.Printf("   POST   /api/content/{id}/rollback\n")
+	fmt.Printf("\n🔄 Press Ctrl+C to shutdown gracefully\n\n")
+
+	// Setup graceful shutdown
+	server := &http.Server{
+		Addr:    addr,
+		Handler: router,
+	}
+
+	// Start server in a goroutine
+	go func() {
+		if err := server.ListenAndServe(); err != nil && err != http.ErrServerClosed {
+			log.Fatalf("Server failed to start: %v", err)
+		}
+	}()
+
+	// Wait for interrupt signal
+	quit := make(chan os.Signal, 1)
+	signal.Notify(quit, syscall.SIGINT, syscall.SIGTERM)
+	<-quit
+
+	fmt.Println("\n🛑 Shutting down server...")
+	if err := server.Close(); err != nil {
+		log.Fatalf("Server forced to shutdown: %v", err)
+	}
+
+	fmt.Println("✅ Server shutdown complete")
+}