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/alert.go

@@ -0,0 +1,93 @@
+package models
+
+import (
+	"database/sql/driver"
+	"encoding/json"
+	"errors"
+	"time"
+)
+
+type AlertType string
+
+const (
+	AlertHighDemandStalePrice AlertType = "high_demand_stale_price"
+	AlertPriceSpike           AlertType = "price_spike"
+	AlertPriceDrop            AlertType = "price_drop"
+	AlertNoRecentQuotes       AlertType = "no_recent_quotes"
+	AlertTrendingNoPrice      AlertType = "trending_no_price"
+)
+
+type AlertSeverity string
+
+const (
+	SeverityLow      AlertSeverity = "low"
+	SeverityMedium   AlertSeverity = "medium"
+	SeverityHigh     AlertSeverity = "high"
+	SeverityCritical AlertSeverity = "critical"
+)
+
+type AlertStatus string
+
+const (
+	AlertStatusNew          AlertStatus = "new"
+	AlertStatusAcknowledged AlertStatus = "acknowledged"
+	AlertStatusResolved     AlertStatus = "resolved"
+	AlertStatusIgnored      AlertStatus = "ignored"
+)
+
+type AlertDetails map[string]interface{}
+
+func (d AlertDetails) Value() (driver.Value, error) {
+	return json.Marshal(d)
+}
+
+func (d *AlertDetails) Scan(value interface{}) error {
+	if value == nil {
+		*d = make(AlertDetails)
+		return nil
+	}
+	bytes, ok := value.([]byte)
+	if !ok {
+		return errors.New("type assertion to []byte failed")
+	}
+	return json.Unmarshal(bytes, d)
+}
+
+type PricingAlert struct {
+	ID        uint          `gorm:"primaryKey;autoIncrement" json:"id"`
+	LotName   string        `gorm:"size:255;not null" json:"lot_name"`
+	AlertType AlertType     `gorm:"type:enum('high_demand_stale_price','price_spike','price_drop','no_recent_quotes','trending_no_price');not null" json:"alert_type"`
+	Severity  AlertSeverity `gorm:"type:enum('low','medium','high','critical');default:'medium'" json:"severity"`
+	Message   string        `gorm:"type:text;not null" json:"message"`
+	Details   AlertDetails  `gorm:"type:json" json:"details"`
+	Status    AlertStatus   `gorm:"type:enum('new','acknowledged','resolved','ignored');default:'new'" json:"status"`
+	CreatedAt time.Time     `gorm:"autoCreateTime" json:"created_at"`
+}
+
+func (PricingAlert) TableName() string {
+	return "qt_pricing_alerts"
+}
+
+type TrendDirection string
+
+const (
+	TrendUp     TrendDirection = "up"
+	TrendStable TrendDirection = "stable"
+	TrendDown   TrendDirection = "down"
+)
+
+type ComponentUsageStats struct {
+	LotName        string         `gorm:"column:lot_name;primaryKey;size:255" json:"lot_name"`
+	QuotesTotal    int            `gorm:"default:0" json:"quotes_total"`
+	QuotesLast30d  int            `gorm:"default:0" json:"quotes_last_30d"`
+	QuotesLast7d   int            `gorm:"default:0" json:"quotes_last_7d"`
+	TotalQuantity  int            `gorm:"default:0" json:"total_quantity"`
+	TotalRevenue   float64        `gorm:"type:decimal(14,2);default:0" json:"total_revenue"`
+	TrendDirection TrendDirection `gorm:"type:enum('up','stable','down');default:'stable'" json:"trend_direction"`
+	TrendPercent   float64        `gorm:"type:decimal(5,2);default:0" json:"trend_percent"`
+	LastUsedAt     *time.Time     `json:"last_used_at"`
+}
+
+func (ComponentUsageStats) TableName() string {
+	return "qt_component_usage_stats"
+}