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/configuration.go

@@ -0,0 +1,74 @@
+package models
+
+import (
+	"database/sql/driver"
+	"encoding/json"
+	"errors"
+	"time"
+)
+
+type ConfigItem struct {
+	LotName   string  `json:"lot_name"`
+	Quantity  int     `json:"quantity"`
+	UnitPrice float64 `json:"unit_price"`
+}
+
+type ConfigItems []ConfigItem
+
+func (c ConfigItems) Value() (driver.Value, error) {
+	return json.Marshal(c)
+}
+
+func (c *ConfigItems) Scan(value interface{}) error {
+	if value == nil {
+		*c = make(ConfigItems, 0)
+		return nil
+	}
+	bytes, ok := value.([]byte)
+	if !ok {
+		return errors.New("type assertion to []byte failed")
+	}
+	return json.Unmarshal(bytes, c)
+}
+
+func (c ConfigItems) Total() float64 {
+	var total float64
+	for _, item := range c {
+		total += item.UnitPrice * float64(item.Quantity)
+	}
+	return total
+}
+
+type Configuration struct {
+	ID         uint        `gorm:"primaryKey;autoIncrement" json:"id"`
+	UUID       string      `gorm:"size:36;uniqueIndex;not null" json:"uuid"`
+	UserID     uint        `gorm:"not null" json:"user_id"`
+	Name       string      `gorm:"size:200;not null" json:"name"`
+	Items      ConfigItems `gorm:"type:json;not null" json:"items"`
+	TotalPrice *float64    `gorm:"type:decimal(12,2)" json:"total_price"`
+	Notes      string      `gorm:"type:text" json:"notes"`
+	IsTemplate bool        `gorm:"default:false" json:"is_template"`
+	CreatedAt  time.Time   `gorm:"autoCreateTime" json:"created_at"`
+
+	User *User `gorm:"foreignKey:UserID" json:"user,omitempty"`
+}
+
+func (Configuration) TableName() string {
+	return "qt_configurations"
+}
+
+type PriceOverride struct {
+	ID         uint       `gorm:"primaryKey;autoIncrement" json:"id"`
+	LotName    string     `gorm:"size:255;not null" json:"lot_name"`
+	Price      float64    `gorm:"type:decimal(12,2);not null" json:"price"`
+	ValidFrom  time.Time  `gorm:"type:date;not null" json:"valid_from"`
+	ValidUntil *time.Time `gorm:"type:date" json:"valid_until"`
+	Reason     string     `gorm:"type:text" json:"reason"`
+	CreatedBy  uint       `gorm:"not null" json:"created_by"`
+
+	Creator *User `gorm:"foreignKey:CreatedBy" json:"creator,omitempty"`
+}
+
+func (PriceOverride) TableName() string {
+	return "qt_price_overrides"
+}