Add initial project structure with core functionality for book and stationery uploads

- Created main application entry point in `main.go`.
- Added configuration management in `config/config.go` and tests in `config/config_test.go`.
- Implemented handlers for book and stationery uploads in `handlers/book.go` and `handlers/stationery.go`, including validation logic.
- Established database connection and services in `services/database.go` and `services/book_service.go`.
- Defined models for books and stationery in `models/book.go` and `models/stationery.go`.
- Set up Firebase integration for image uploads in `services/firebase.go`.
- Created migration scripts for database schema in `migrations/001_create_tables.sql` and subsequent updates.
- Added CORS and error handling middleware.
- Included comprehensive tests for handlers, services, and utilities.
- Documented API endpoints and usage in `README.md` and migration instructions in `migrations/README.md`.
- Introduced `.gitignore` to exclude unnecessary files and directories.
- Added Go module support with `go.mod` and `go.sum` files.
- Implemented utility functions for slug generation and validation in `utils/slug.go` and `utils/validation.go`.

services/firebase.go

@@ -0,0 +1,141 @@
+package services
+
+import (
+	"context"
+	"fmt"
+	"os"
+	"path/filepath"
+	"time"
+
+	"jd-book-uploader/config"
+
+	firebase "firebase.google.com/go/v4"
+	"firebase.google.com/go/v4/storage"
+	"google.golang.org/api/option"
+)
+
+var (
+	FirebaseApp    *firebase.App
+	FirebaseClient *storage.Client
+)
+
+// InitFirebase initializes Firebase Admin SDK and Storage client
+func InitFirebase(cfg *config.Config) (*storage.Client, error) {
+	ctx := context.Background()
+
+	// Determine credentials file path
+	credentialsFile := cfg.FirebaseCredentialsFile
+	if credentialsFile == "" {
+		// Fallback to GOOGLE_APPLICATION_CREDENTIALS env var
+		credentialsFile = os.Getenv("GOOGLE_APPLICATION_CREDENTIALS")
+	}
+
+	// Check if credentials file exists
+	if credentialsFile != "" {
+		if _, err := os.Stat(credentialsFile); os.IsNotExist(err) {
+			return nil, fmt.Errorf("firebase credentials file not found: %s", credentialsFile)
+		}
+		// Convert relative path to absolute
+		if !filepath.IsAbs(credentialsFile) {
+			credentialsFile, _ = filepath.Abs(credentialsFile)
+		}
+	}
+
+	// Build Firebase config
+	firebaseConfig := &firebase.Config{
+		ProjectID:     cfg.FirebaseProjectID,
+		StorageBucket: cfg.FirebaseStorageBucket,
+	}
+
+	// Initialize Firebase app
+	var app *firebase.App
+	var err error
+
+	if credentialsFile != "" {
+		// Use credentials file
+		opt := option.WithCredentialsFile(credentialsFile)
+		app, err = firebase.NewApp(ctx, firebaseConfig, opt)
+	} else {
+		// Use default credentials (from environment)
+		app, err = firebase.NewApp(ctx, firebaseConfig)
+	}
+
+	if err != nil {
+		return nil, fmt.Errorf("failed to initialize Firebase app: %w", err)
+	}
+
+	// Create Storage client
+	client, err := app.Storage(ctx)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create Firebase Storage client: %w", err)
+	}
+
+	FirebaseApp = app
+	FirebaseClient = client
+
+	return client, nil
+}
+
+// UploadImage uploads an image to Firebase Storage and returns the public URL
+// folderPath: storage folder path (e.g., "/jd-bookshop/books" or "/jd-bookshop/stationery")
+// filename: image filename (should be the slug with .png extension)
+func UploadImage(ctx context.Context, imageData []byte, folderPath string, filename string) (string, error) {
+	if FirebaseClient == nil {
+		return "", fmt.Errorf("firebase client not initialized")
+	}
+
+	// Build object path: {folderPath}/{filename}
+	// Use provided path strictly without adding year/month
+	objectPath := fmt.Sprintf("%s/%s", folderPath, filename)
+
+	// Get bucket handle
+	bucket, err := FirebaseClient.DefaultBucket()
+	if err != nil {
+		return "", fmt.Errorf("failed to get default bucket: %w", err)
+	}
+
+	// Create object handle
+	obj := bucket.Object(objectPath)
+
+	// Create writer
+	writer := obj.NewWriter(ctx)
+	writer.ContentType = "image/png"
+	writer.CacheControl = "public, max-age=31536000" // 1 year cache
+
+	// Write image data with retry logic
+	maxRetries := 3
+	var writeErr error
+	for i := 0; i < maxRetries; i++ {
+		if i > 0 {
+			// Wait before retry (exponential backoff)
+			time.Sleep(time.Duration(i) * time.Second)
+		}
+
+		// Write data
+		_, writeErr = writer.Write(imageData)
+		if writeErr == nil {
+			break
+		}
+	}
+
+	if writeErr != nil {
+		writer.Close()
+		return "", fmt.Errorf("failed to write image data after %d attempts: %w", maxRetries, writeErr)
+	}
+
+	// Close writer to finalize upload
+	if err := writer.Close(); err != nil {
+		return "", fmt.Errorf("failed to close writer: %w", err)
+	}
+
+	// Get public URL from Firebase
+	attrs, err := obj.Attrs(ctx)
+	if err != nil {
+		return "", fmt.Errorf("failed to get object attributes: %w", err)
+	}
+
+	// Use Firebase's original download link (MediaLink)
+	publicURL := attrs.MediaLink
+
+	return publicURL, nil
+}