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/category.go

@@ -0,0 +1,38 @@
+package repository
+
+import (
+	"github.com/mchus/quoteforge/internal/models"
+	"gorm.io/gorm"
+)
+
+type CategoryRepository struct {
+	db *gorm.DB
+}
+
+func NewCategoryRepository(db *gorm.DB) *CategoryRepository {
+	return &CategoryRepository{db: db}
+}
+
+func (r *CategoryRepository) GetAll() ([]models.Category, error) {
+	var categories []models.Category
+	err := r.db.Order("display_order ASC").Find(&categories).Error
+	return categories, err
+}
+
+func (r *CategoryRepository) GetByCode(code string) (*models.Category, error) {
+	var category models.Category
+	err := r.db.Where("code = ?", code).First(&category).Error
+	if err != nil {
+		return nil, err
+	}
+	return &category, nil
+}
+
+func (r *CategoryRepository) GetByID(id uint) (*models.Category, error) {
+	var category models.Category
+	err := r.db.First(&category, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &category, nil
+}