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/configuration.go

@@ -0,0 +1,75 @@
+package repository
+
+import (
+	"github.com/mchus/quoteforge/internal/models"
+	"gorm.io/gorm"
+)
+
+type ConfigurationRepository struct {
+	db *gorm.DB
+}
+
+func NewConfigurationRepository(db *gorm.DB) *ConfigurationRepository {
+	return &ConfigurationRepository{db: db}
+}
+
+func (r *ConfigurationRepository) Create(config *models.Configuration) error {
+	return r.db.Create(config).Error
+}
+
+func (r *ConfigurationRepository) GetByID(id uint) (*models.Configuration, error) {
+	var config models.Configuration
+	err := r.db.Preload("User").First(&config, id).Error
+	if err != nil {
+		return nil, err
+	}
+	return &config, nil
+}
+
+func (r *ConfigurationRepository) GetByUUID(uuid string) (*models.Configuration, error) {
+	var config models.Configuration
+	err := r.db.Preload("User").Where("uuid = ?", uuid).First(&config).Error
+	if err != nil {
+		return nil, err
+	}
+	return &config, nil
+}
+
+func (r *ConfigurationRepository) Update(config *models.Configuration) error {
+	return r.db.Save(config).Error
+}
+
+func (r *ConfigurationRepository) Delete(id uint) error {
+	return r.db.Delete(&models.Configuration{}, id).Error
+}
+
+func (r *ConfigurationRepository) ListByUser(userID uint, offset, limit int) ([]models.Configuration, int64, error) {
+	var configs []models.Configuration
+	var total int64
+
+	r.db.Model(&models.Configuration{}).Where("user_id = ?", userID).Count(&total)
+	err := r.db.
+		Where("user_id = ?", userID).
+		Order("created_at DESC").
+		Offset(offset).
+		Limit(limit).
+		Find(&configs).Error
+
+	return configs, total, err
+}
+
+func (r *ConfigurationRepository) ListTemplates(offset, limit int) ([]models.Configuration, int64, error) {
+	var configs []models.Configuration
+	var total int64
+
+	r.db.Model(&models.Configuration{}).Where("is_template = ?", true).Count(&total)
+	err := r.db.
+		Preload("User").
+		Where("is_template = ?", true).
+		Order("created_at DESC").
+		Offset(offset).
+		Limit(limit).
+		Find(&configs).Error
+
+	return configs, total, err
+}