Add initial backend implementation

- Go module with Gin, GORM, JWT, excelize dependencies
- Configuration loading from YAML with all settings
- GORM models for users, categories, components, configurations, alerts
- Repository layer for all entities
- Services: auth (JWT), pricing (median/average/weighted), components,
  quotes, configurations, export (CSV/XLSX), alerts
- Middleware: JWT auth, role-based access, CORS
- HTTP handlers for all API endpoints
- Main server with dependency injection and graceful shutdown

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

internal/repository/user.go

@@ -0,0 +1,62 @@
+package repository
+
+import (
+	"github.com/mchus/quoteforge/internal/models"
+	"gorm.io/gorm"
+)
+
+type UserRepository struct {
+	db *gorm.DB
+}
+
+func NewUserRepository(db *gorm.DB) *UserRepository {
+	return &UserRepository{db: db}
+}
+
+func (r *UserRepository) Create(user *models.User) error {
+	return r.db.Create(user).Error
+}
+
+func (r *UserRepository) GetByID(id uint) (*models.User, error) {
+	var user models.User
+	err := r.db.First(&user, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+func (r *UserRepository) GetByUsername(username string) (*models.User, error) {
+	var user models.User
+	err := r.db.Where("username = ?", username).First(&user).Error
+	if err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+func (r *UserRepository) GetByEmail(email string) (*models.User, error) {
+	var user models.User
+	err := r.db.Where("email = ?", email).First(&user).Error
+	if err != nil {
+		return nil, err
+	}
+	return &user, nil
+}
+
+func (r *UserRepository) Update(user *models.User) error {
+	return r.db.Save(user).Error
+}
+
+func (r *UserRepository) Delete(id uint) error {
+	return r.db.Delete(&models.User{}, id).Error
+}
+
+func (r *UserRepository) List(offset, limit int) ([]models.User, int64, error) {
+	var users []models.User
+	var total int64
+
+	r.db.Model(&models.User{}).Count(&total)
+	err := r.db.Offset(offset).Limit(limit).Find(&users).Error
+	return users, total, err
+}