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/database.go

@@ -0,0 +1,85 @@
+package services
+
+import (
+	"context"
+	"fmt"
+	"time"
+
+	"github.com/jackc/pgx/v5/pgxpool"
+	"jd-book-uploader/config"
+)
+
+// DB holds the database connection pool
+var DB *pgxpool.Pool
+
+// NewDBPool creates a new database connection pool
+func NewDBPool(cfg *config.Config) (*pgxpool.Pool, error) {
+	// Build connection string
+	connString := fmt.Sprintf(
+		"host=%s port=%s user=%s password=%s dbname=%s sslmode=disable",
+		cfg.DBHost,
+		cfg.DBPort,
+		cfg.DBUser,
+		cfg.DBPassword,
+		cfg.DBName,
+	)
+
+	// Parse connection string
+	poolConfig, err := pgxpool.ParseConfig(connString)
+	if err != nil {
+		return nil, fmt.Errorf("failed to parse connection string: %w", err)
+	}
+
+	// Configure connection pool
+	poolConfig.MaxConns = 10
+	poolConfig.MinConns = 2
+	poolConfig.MaxConnLifetime = time.Hour
+	poolConfig.MaxConnIdleTime = time.Minute * 30
+	poolConfig.HealthCheckPeriod = time.Minute
+
+	// Set connection timeout
+	ctx, cancel := context.WithTimeout(context.Background(), 5*time.Second)
+	defer cancel()
+
+	// Create connection pool
+	pool, err := pgxpool.NewWithConfig(ctx, poolConfig)
+	if err != nil {
+		return nil, fmt.Errorf("failed to create connection pool: %w", err)
+	}
+
+	// Test connection
+	if err := pool.Ping(ctx); err != nil {
+		pool.Close()
+		return nil, fmt.Errorf("failed to ping database: %w", err)
+	}
+
+	DB = pool
+	return pool, nil
+}
+
+// CloseDB closes the database connection pool
+func CloseDB() error {
+	if DB != nil {
+		DB.Close()
+	}
+	return nil
+}
+
+// RetryConnection attempts to reconnect to the database with retries
+func RetryConnection(cfg *config.Config, maxRetries int, retryDelay time.Duration) (*pgxpool.Pool, error) {
+	var pool *pgxpool.Pool
+	var err error
+
+	for i := 0; i < maxRetries; i++ {
+		pool, err = NewDBPool(cfg)
+		if err == nil {
+			return pool, nil
+		}
+
+		if i < maxRetries-1 {
+			time.Sleep(retryDelay)
+		}
+	}
+
+	return nil, fmt.Errorf("failed to connect after %d attempts: %w", maxRetries, err)
+}