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/client.go

@@ -0,0 +1,164 @@
+package content
+
+import (
+	"encoding/json"
+	"fmt"
+	"io"
+	"net/http"
+	"net/url"
+	"strings"
+	"time"
+)
+
+// HTTPClient implements ContentClient for HTTP API access
+type HTTPClient struct {
+	BaseURL    string
+	APIKey     string
+	HTTPClient *http.Client
+}
+
+// NewHTTPClient creates a new HTTP content client
+func NewHTTPClient(baseURL, apiKey string) *HTTPClient {
+	return &HTTPClient{
+		BaseURL: strings.TrimSuffix(baseURL, "/"),
+		APIKey:  apiKey,
+		HTTPClient: &http.Client{
+			Timeout: 30 * time.Second,
+		},
+	}
+}
+
+// GetContent fetches a single content item by ID
+func (c *HTTPClient) GetContent(siteID, contentID string) (*ContentItem, error) {
+	url := fmt.Sprintf("%s/api/content/%s?site_id=%s", c.BaseURL, contentID, siteID)
+
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		return nil, fmt.Errorf("creating request: %w", err)
+	}
+
+	if c.APIKey != "" {
+		req.Header.Set("Authorization", "Bearer "+c.APIKey)
+	}
+
+	resp, err := c.HTTPClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("making request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode == 404 {
+		return nil, nil // Content not found, return nil without error
+	}
+
+	if resp.StatusCode != 200 {
+		return nil, fmt.Errorf("API error: %s", resp.Status)
+	}
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("reading response: %w", err)
+	}
+
+	var item ContentItem
+	if err := json.Unmarshal(body, &item); err != nil {
+		return nil, fmt.Errorf("parsing response: %w", err)
+	}
+
+	return &item, nil
+}
+
+// GetBulkContent fetches multiple content items by IDs
+func (c *HTTPClient) GetBulkContent(siteID string, contentIDs []string) (map[string]ContentItem, error) {
+	if len(contentIDs) == 0 {
+		return make(map[string]ContentItem), nil
+	}
+
+	// Build query parameters
+	params := url.Values{}
+	params.Set("site_id", siteID)
+	for _, id := range contentIDs {
+		params.Add("ids", id)
+	}
+
+	url := fmt.Sprintf("%s/api/content/bulk?%s", c.BaseURL, params.Encode())
+
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		return nil, fmt.Errorf("creating request: %w", err)
+	}
+
+	if c.APIKey != "" {
+		req.Header.Set("Authorization", "Bearer "+c.APIKey)
+	}
+
+	resp, err := c.HTTPClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("making request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != 200 {
+		return nil, fmt.Errorf("API error: %s", resp.Status)
+	}
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("reading response: %w", err)
+	}
+
+	var response ContentResponse
+	if err := json.Unmarshal(body, &response); err != nil {
+		return nil, fmt.Errorf("parsing response: %w", err)
+	}
+
+	// Convert slice to map for easy lookup
+	result := make(map[string]ContentItem)
+	for _, item := range response.Content {
+		result[item.ID] = item
+	}
+
+	return result, nil
+}
+
+// GetAllContent fetches all content for a site
+func (c *HTTPClient) GetAllContent(siteID string) (map[string]ContentItem, error) {
+	url := fmt.Sprintf("%s/api/content?site_id=%s", c.BaseURL, siteID)
+
+	req, err := http.NewRequest("GET", url, nil)
+	if err != nil {
+		return nil, fmt.Errorf("creating request: %w", err)
+	}
+
+	if c.APIKey != "" {
+		req.Header.Set("Authorization", "Bearer "+c.APIKey)
+	}
+
+	resp, err := c.HTTPClient.Do(req)
+	if err != nil {
+		return nil, fmt.Errorf("making request: %w", err)
+	}
+	defer resp.Body.Close()
+
+	if resp.StatusCode != 200 {
+		return nil, fmt.Errorf("API error: %s", resp.Status)
+	}
+
+	body, err := io.ReadAll(resp.Body)
+	if err != nil {
+		return nil, fmt.Errorf("reading response: %w", err)
+	}
+
+	var response ContentResponse
+	if err := json.Unmarshal(body, &response); err != nil {
+		return nil, fmt.Errorf("parsing response: %w", err)
+	}
+
+	// Convert slice to map for easy lookup
+	result := make(map[string]ContentItem)
+	for _, item := range response.Content {
+		result[item.ID] = item
+	}
+
+	return result, nil
+}