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/models.go

@@ -0,0 +1,32 @@
+package models
+
+import "gorm.io/gorm"
+
+// AllModels returns all models for auto-migration
+func AllModels() []interface{} {
+	return []interface{}{
+		&User{},
+		&Category{},
+		&LotMetadata{},
+		&Configuration{},
+		&PriceOverride{},
+		&PricingAlert{},
+		&ComponentUsageStats{},
+	}
+}
+
+// Migrate runs auto-migration for all QuoteForge tables
+func Migrate(db *gorm.DB) error {
+	return db.AutoMigrate(AllModels()...)
+}
+
+// SeedCategories inserts default categories if not exist
+func SeedCategories(db *gorm.DB) error {
+	for _, cat := range DefaultCategories {
+		result := db.Where("code = ?", cat.Code).FirstOrCreate(&cat)
+		if result.Error != nil {
+			return result.Error
+		}
+	}
+	return nil
+}