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`.

config/config.go

@@ -0,0 +1,107 @@
+package config
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/joho/godotenv"
+)
+
+// Config holds all application configuration
+type Config struct {
+	// Server
+	Port        string
+	FrontendURL string
+
+	// Database
+	DBHost     string
+	DBPort     string
+	DBUser     string
+	DBPassword string
+	DBName     string
+
+	// Firebase
+	FirebaseProjectID       string
+	FirebaseStorageBucket   string
+	FirebaseCredentialsFile string
+}
+
+// LoadConfig loads configuration from environment variables
+// Loading order (later files override earlier ones):
+// 1. .env (base/default values)
+// 2. .env.local (local development overrides - takes precedence)
+// Note: OS environment variables take highest precedence (godotenv won't override them)
+func LoadConfig() (*Config, error) {
+	// Load .env file if it exists (optional, won't error if missing)
+	_ = godotenv.Load(".env")
+
+	// Load .env.local if it exists (overrides .env values)
+	// This allows local development overrides without modifying .env
+	_ = godotenv.Load(".env.local")
+
+	cfg := &Config{
+		// Server
+		Port:        getEnv("PORT", "8080"),
+		FrontendURL: getEnv("FRONTEND_URL", "http://localhost:5173"),
+
+		// Database
+		DBHost:     getEnv("DB_HOST", "localhost"),
+		DBPort:     getEnv("DB_PORT", "5432"),
+		DBUser:     getEnv("DB_USER", ""),
+		DBPassword: getEnv("DB_PASSWORD", ""),
+		DBName:     getEnv("DB_NAME", ""),
+
+		// Firebase
+		FirebaseProjectID:       getEnv("FIREBASE_PROJECT_ID", ""),
+		FirebaseStorageBucket:   getEnv("FIREBASE_STORAGE_BUCKET", ""),
+		FirebaseCredentialsFile: getEnv("FIREBASE_CREDENTIALS_FILE", ""),
+	}
+
+	// Validate required fields
+	if err := cfg.Validate(); err != nil {
+		return nil, fmt.Errorf("config validation failed: %w", err)
+	}
+
+	return cfg, nil
+}
+
+// Validate checks that all required configuration values are set
+func (c *Config) Validate() error {
+	var missing []string
+
+	if c.DBUser == "" {
+		missing = append(missing, "DB_USER")
+	}
+	if c.DBPassword == "" {
+		missing = append(missing, "DB_PASSWORD")
+	}
+	if c.DBName == "" {
+		missing = append(missing, "DB_NAME")
+	}
+	if c.FirebaseProjectID == "" {
+		missing = append(missing, "FIREBASE_PROJECT_ID")
+	}
+	if c.FirebaseStorageBucket == "" {
+		missing = append(missing, "FIREBASE_STORAGE_BUCKET")
+	}
+	if c.FirebaseCredentialsFile == "" {
+		// Check if GOOGLE_APPLICATION_CREDENTIALS is set as fallback
+		if os.Getenv("GOOGLE_APPLICATION_CREDENTIALS") == "" {
+			missing = append(missing, "FIREBASE_CREDENTIALS_FILE (or GOOGLE_APPLICATION_CREDENTIALS)")
+		}
+	}
+
+	if len(missing) > 0 {
+		return fmt.Errorf("missing required environment variables: %v", missing)
+	}
+
+	return nil
+}
+
+// getEnv gets an environment variable or returns a default value
+func getEnv(key, defaultValue string) string {
+	if value := os.Getenv(key); value != "" {
+		return value
+	}
+	return defaultValue
+}