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/models/user.go

@@ -0,0 +1,39 @@
+package models
+
+import "time"
+
+type UserRole string
+
+const (
+	RoleViewer       UserRole = "viewer"
+	RoleEditor       UserRole = "editor"
+	RolePricingAdmin UserRole = "pricing_admin"
+	RoleAdmin        UserRole = "admin"
+)
+
+type User struct {
+	ID           uint      `gorm:"primaryKey;autoIncrement" json:"id"`
+	Username     string    `gorm:"size:100;uniqueIndex;not null" json:"username"`
+	Email        string    `gorm:"size:255;uniqueIndex;not null" json:"email"`
+	PasswordHash string    `gorm:"size:255;not null" json:"-"`
+	Role         UserRole  `gorm:"type:enum('viewer','editor','pricing_admin','admin');default:'viewer'" json:"role"`
+	IsActive     bool      `gorm:"default:true" json:"is_active"`
+	CreatedAt    time.Time `gorm:"autoCreateTime" json:"created_at"`
+	UpdatedAt    time.Time `gorm:"autoUpdateTime" json:"updated_at"`
+}
+
+func (User) TableName() string {
+	return "qt_users"
+}
+
+func (u *User) CanEdit() bool {
+	return u.Role == RoleEditor || u.Role == RolePricingAdmin || u.Role == RoleAdmin
+}
+
+func (u *User) CanManagePricing() bool {
+	return u.Role == RolePricingAdmin || u.Role == RoleAdmin
+}
+
+func (u *User) CanManageUsers() bool {
+	return u.Role == RoleAdmin
+}