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/mock.go

@@ -0,0 +1,138 @@
+package content
+
+import (
+	"time"
+)
+
+// MockClient implements ContentClient with mock data for development
+type MockClient struct {
+	data map[string]ContentItem
+}
+
+// NewMockClient creates a new mock content client with sample data
+func NewMockClient() *MockClient {
+	// Generate realistic mock content based on actual generated IDs
+	data := map[string]ContentItem{
+		// Navigation (index.html has collision suffix)
+		"navbar-logo-2b10ad": {
+			ID:        "navbar-logo-2b10ad",
+			SiteID:    "demo",
+			Value:     "Acme Consulting Solutions",
+			Type:      "text",
+			UpdatedAt: time.Now().Format(time.RFC3339),
+		},
+		"navbar-logo-2b10ad-a44bad": {
+			ID:        "navbar-logo-2b10ad-a44bad",
+			SiteID:    "demo",
+			Value:     "Acme Business Advisors",
+			Type:      "text",
+			UpdatedAt: time.Now().Format(time.RFC3339),
+		},
+
+		// Hero Section - index.html (updated with actual IDs)
+		"hero-title-7cfeea": {
+			ID:        "hero-title-7cfeea",
+			SiteID:    "demo",
+			Value:     "Transform Your Business with Strategic Expertise",
+			Type:      "text",
+			UpdatedAt: time.Now().Format(time.RFC3339),
+		},
+		"hero-lead-e47475": {
+			ID:        "hero-lead-e47475",
+			SiteID:    "demo",
+			Value:     "We help **ambitious businesses** grow through strategic planning, process optimization, and digital transformation. Our team brings 20+ years of experience to accelerate your success.",
+			Type:      "markdown",
+			UpdatedAt: time.Now().Format(time.RFC3339),
+		},
+		"hero-link-76c620": {
+			ID:        "hero-link-76c620",
+			SiteID:    "demo",
+			Value:     "Schedule Free Consultation",
+			Type:      "link",
+			UpdatedAt: time.Now().Format(time.RFC3339),
+		},
+
+		// Hero Section - about.html
+		"hero-title-c70343": {
+			ID:        "hero-title-c70343",
+			SiteID:    "demo",
+			Value:     "About Our Consulting Expertise",
+			Type:      "text",
+			UpdatedAt: time.Now().Format(time.RFC3339),
+		},
+		"hero-lead-673026": {
+			ID:        "hero-lead-673026",
+			SiteID:    "demo",
+			Value:     "We're a team of **experienced consultants** dedicated to helping small businesses thrive in today's competitive marketplace through proven strategies.",
+			Type:      "markdown",
+			UpdatedAt: time.Now().Format(time.RFC3339),
+		},
+
+		// Services Section
+		"services-subtitle-c8927c": {
+			ID:        "services-subtitle-c8927c",
+			SiteID:    "demo",
+			Value:     "Our Story",
+			Type:      "text",
+			UpdatedAt: time.Now().Format(time.RFC3339),
+		},
+		"services-text-0d96da": {
+			ID:        "services-text-0d96da",
+			SiteID:    "demo",
+			Value:     "**Founded in 2020**, Acme Consulting emerged from a simple observation: small businesses needed access to the same high-quality strategic advice that large corporations receive, but in a format that was accessible, affordable, and actionable.",
+			Type:      "markdown",
+			UpdatedAt: time.Now().Format(time.RFC3339),
+		},
+
+		// Default fallback for any missing content
+		"default": {
+			ID:        "default",
+			SiteID:    "demo",
+			Value:     "[Enhanced Content]",
+			Type:      "text",
+			UpdatedAt: time.Now().Format(time.RFC3339),
+		},
+	}
+
+	return &MockClient{data: data}
+}
+
+// GetContent fetches a single content item by ID
+func (m *MockClient) GetContent(siteID, contentID string) (*ContentItem, error) {
+	if item, exists := m.data[contentID]; exists && item.SiteID == siteID {
+		return &item, nil
+	}
+
+	// Return nil for missing content - this will preserve original HTML content
+	return nil, nil
+}
+
+// GetBulkContent fetches multiple content items by IDs
+func (m *MockClient) GetBulkContent(siteID string, contentIDs []string) (map[string]ContentItem, error) {
+	result := make(map[string]ContentItem)
+
+	for _, id := range contentIDs {
+		item, err := m.GetContent(siteID, id)
+		if err != nil {
+			return nil, err
+		}
+		if item != nil {
+			result[id] = *item
+		}
+	}
+
+	return result, nil
+}
+
+// GetAllContent fetches all content for a site
+func (m *MockClient) GetAllContent(siteID string) (map[string]ContentItem, error) {
+	result := make(map[string]ContentItem)
+
+	for _, item := range m.data {
+		if item.SiteID == siteID {
+			result[item.ID] = item
+		}
+	}
+
+	return result, nil
+}