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/lot.go

@@ -0,0 +1,38 @@
+package models
+
+import "time"
+
+// Lot represents existing lot table (READ-ONLY)
+type Lot struct {
+	LotName        string `gorm:"column:lot_name;primaryKey;size:255"`
+	LotDescription string `gorm:"column:lot_description;size:10000"`
+}
+
+func (Lot) TableName() string {
+	return "lot"
+}
+
+// LotLog represents existing lot_log table (READ-ONLY)
+type LotLog struct {
+	LotLogID uint      `gorm:"column:lot_log_id;primaryKey;autoIncrement"`
+	Lot      string    `gorm:"column:lot;size:255;not null"`
+	Supplier string    `gorm:"column:supplier;size:255;not null"`
+	Date     time.Time `gorm:"column:date;type:date;not null"`
+	Price    float64   `gorm:"column:price;not null"`
+	Quality  string    `gorm:"column:quality;size:255"`
+	Comments string    `gorm:"column:comments;size:15000"`
+}
+
+func (LotLog) TableName() string {
+	return "lot_log"
+}
+
+// Supplier represents existing supplier table (READ-ONLY)
+type Supplier struct {
+	SupplierName    string `gorm:"column:supplier_name;primaryKey;size:255"`
+	SupplierComment string `gorm:"column:supplier_comment;size:10000"`
+}
+
+func (Supplier) TableName() string {
+	return "supplier"
+}