refactor: restructure project from bookmann to shelf

- Rename project from 'bookmann' to 'shelf'
- Move all backend/ contents to root level (flatten structure)
- Update Go module name from 'bookmann' to 'shelf'
- Update all import paths to use new 'shelf' module
- Update Dockerfile to work without backend/ subdirectory
- Update docker-compose.yml to use new structure and rename containers
- Update .gitignore for new file paths
- Update README.md with new project name and structure
- Regenerate database code with new module imports

internal/config/config.go

@@ -0,0 +1,42 @@
+package config
+
+import (
+	"fmt"
+	"os"
+)
+
+type Config struct {
+	ServerPort       string
+	JWTSecret        string
+	UploadPath       string
+	DatabaseHost     string
+	DatabasePort     string
+	DatabaseUser     string
+	DatabasePassword string
+	DatabaseName     string
+}
+
+func LoadConfig() *Config {
+	return &Config{
+		ServerPort:       getEnv("SERVER_PORT", "8080"),
+		DatabaseHost:     getEnv("DATABASE_HOST", "localhost"),
+		DatabasePort:     getEnv("DATABASE_PORT", "5432"),
+		DatabaseUser:     getEnv("DATABASE_USER", "postgres"),
+		DatabasePassword: getEnv("DATABASE_PASSWORD", "password"),
+		DatabaseName:     getEnv("DATABASE_NAME", "ebookdb"),
+		JWTSecret:        getEnv("JWT_SECRET", "your-secret-key"),
+		UploadPath:       getEnv("UPLOAD_PATH", "./uploads"),
+	}
+}
+
+func (c *Config) DatabaseURL() string {
+	return fmt.Sprintf("postgres://%s:%s@%s:%s/%s?sslmode=disable",
+		c.DatabaseUser, c.DatabasePassword, c.DatabaseHost, c.DatabasePort, c.DatabaseName)
+}
+
+func getEnv(key, defaultValue string) string {
+	if value := os.Getenv(key); value != "" {
+		return value
+	}
+	return defaultValue
+}