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/enhance.go

@@ -0,0 +1,82 @@
+package cmd
+
+import (
+	"fmt"
+	"log"
+	"os"
+
+	"github.com/spf13/cobra"
+	"github.com/spf13/viper"
+
+	"github.com/insertr/insertr/internal/content"
+)
+
+var enhanceCmd = &cobra.Command{
+	Use:   "enhance [input-dir]",
+	Short: "Enhance HTML files by injecting content from database",
+	Long: `Enhance processes HTML files and injects latest content from the database
+while adding editing capabilities. This is the core build-time enhancement
+process that transforms static HTML into an editable CMS.`,
+	Args: cobra.ExactArgs(1),
+	Run:  runEnhance,
+}
+
+var (
+	outputDir   string
+	mockContent bool
+)
+
+func init() {
+	enhanceCmd.Flags().StringVarP(&outputDir, "output", "o", "./dist", "Output directory for enhanced files")
+	enhanceCmd.Flags().BoolVar(&mockContent, "mock", true, "Use mock content for development")
+
+	// Bind flags to viper
+	viper.BindPFlag("build.output", enhanceCmd.Flags().Lookup("output"))
+	viper.BindPFlag("mock_content", enhanceCmd.Flags().Lookup("mock"))
+}
+
+func runEnhance(cmd *cobra.Command, args []string) {
+	inputDir := args[0]
+
+	// Validate input directory
+	if _, err := os.Stat(inputDir); os.IsNotExist(err) {
+		log.Fatalf("Input directory does not exist: %s", inputDir)
+	}
+
+	// Get configuration values
+	dbPath := viper.GetString("database.path")
+	apiURL := viper.GetString("api.url")
+	apiKey := viper.GetString("api.key")
+	siteID := viper.GetString("site_id")
+	mockContent := viper.GetBool("mock_content")
+
+	// Create content client
+	var client content.ContentClient
+	if mockContent || (apiURL == "" && dbPath == "") {
+		fmt.Printf("🧪 Using mock content for development\n")
+		client = content.NewMockClient()
+	} else if apiURL != "" {
+		fmt.Printf("🌐 Using content API: %s\n", apiURL)
+		client = content.NewHTTPClient(apiURL, apiKey)
+	} else {
+		fmt.Printf("🗄️ Using database: %s\n", dbPath)
+		// TODO: Implement database client for direct DB access
+		fmt.Printf("⚠️  Direct database access not yet implemented, using mock content\n")
+		client = content.NewMockClient()
+	}
+
+	// Create enhancer
+	enhancer := content.NewEnhancer(client, siteID)
+
+	fmt.Printf("🚀 Starting enhancement process...\n")
+	fmt.Printf("📁 Input: %s\n", inputDir)
+	fmt.Printf("📁 Output: %s\n", outputDir)
+	fmt.Printf("🏷️  Site ID: %s\n\n", siteID)
+
+	// Enhance directory
+	if err := enhancer.EnhanceDirectory(inputDir, outputDir); err != nil {
+		log.Fatalf("Enhancement failed: %v", err)
+	}
+
+	fmt.Printf("\n✅ Enhancement complete! Enhanced files available in: %s\n", outputDir)
+}