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

internal/content/library.go

@@ -0,0 +1,50 @@
+package content
+
+import (
+	_ "embed"
+	"fmt"
+)
+
+// Embedded library assets
+//
+//go:embed assets/insertr.min.js
+var libraryMinJS string
+
+//go:embed assets/insertr.js
+var libraryJS string
+
+// GetLibraryScript returns the appropriate library version
+func GetLibraryScript(minified bool) string {
+	if minified {
+		return libraryMinJS
+	}
+	return libraryJS
+}
+
+// GetLibraryVersion returns the current embedded library version
+func GetLibraryVersion() string {
+	return "1.0.0"
+}
+
+// GetLibraryURL returns the appropriate library URL for script injection
+func GetLibraryURL(minified bool, isDevelopment bool) string {
+	if isDevelopment {
+		// Local development URLs - relative to served content
+		if minified {
+			return "/insertr/insertr.min.js"
+		}
+		return "/insertr/insertr.js"
+	}
+
+	// Production URLs - use CDN
+	return GetLibraryCDNURL(minified)
+}
+
+// GetLibraryCDNURL returns the CDN URL for production use
+func GetLibraryCDNURL(minified bool) string {
+	version := GetLibraryVersion()
+	if minified {
+		return fmt.Sprintf("https://cdn.jsdelivr.net/npm/@insertr/lib@%s/dist/insertr.min.js", version)
+	}
+	return fmt.Sprintf("https://cdn.jsdelivr.net/npm/@insertr/lib@%s/dist/insertr.js", version)
+}