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/services/pricing/service.go

@@ -0,0 +1,178 @@
+package pricing
+
+import (
+	"time"
+
+	"github.com/mchus/quoteforge/internal/config"
+	"github.com/mchus/quoteforge/internal/models"
+	"github.com/mchus/quoteforge/internal/repository"
+)
+
+type Service struct {
+	componentRepo *repository.ComponentRepository
+	priceRepo     *repository.PriceRepository
+	config        config.PricingConfig
+}
+
+func NewService(
+	componentRepo *repository.ComponentRepository,
+	priceRepo *repository.PriceRepository,
+	cfg config.PricingConfig,
+) *Service {
+	return &Service{
+		componentRepo: componentRepo,
+		priceRepo:     priceRepo,
+		config:        cfg,
+	}
+}
+
+// GetEffectivePrice returns the current effective price for a component
+// Priority: active override > calculated price > nil
+func (s *Service) GetEffectivePrice(lotName string) (*float64, error) {
+	// Check for active override first
+	override, err := s.priceRepo.GetPriceOverride(lotName)
+	if err == nil && override != nil {
+		return &override.Price, nil
+	}
+
+	// Get component metadata
+	component, err := s.componentRepo.GetByLotName(lotName)
+	if err != nil {
+		return nil, err
+	}
+
+	return component.CurrentPrice, nil
+}
+
+// CalculatePrice calculates price using the specified method
+func (s *Service) CalculatePrice(lotName string, method models.PriceMethod, periodDays int) (float64, error) {
+	if periodDays == 0 {
+		periodDays = s.config.DefaultPeriodDays
+	}
+
+	points, err := s.priceRepo.GetPriceHistory(lotName, periodDays)
+	if err != nil {
+		return 0, err
+	}
+
+	if len(points) == 0 {
+		return 0, nil
+	}
+
+	prices := make([]float64, len(points))
+	for i, p := range points {
+		prices[i] = p.Price
+	}
+
+	switch method {
+	case models.PriceMethodAverage:
+		return CalculateAverage(prices), nil
+	case models.PriceMethodWeightedMedian:
+		return CalculateWeightedMedian(points, s.config.DefaultPeriodDays), nil
+	case models.PriceMethodMedian:
+		fallthrough
+	default:
+		return CalculateMedian(prices), nil
+	}
+}
+
+// UpdateComponentPrice recalculates and updates the price for a component
+func (s *Service) UpdateComponentPrice(lotName string) error {
+	component, err := s.componentRepo.GetByLotName(lotName)
+	if err != nil {
+		return err
+	}
+
+	price, err := s.CalculatePrice(lotName, component.PriceMethod, component.PricePeriodDays)
+	if err != nil {
+		return err
+	}
+
+	now := time.Now()
+	if price > 0 {
+		component.CurrentPrice = &price
+		component.PriceUpdatedAt = &now
+	}
+
+	return s.componentRepo.Update(component)
+}
+
+// SetManualPrice sets a manual price override
+func (s *Service) SetManualPrice(lotName string, price float64, reason string, userID uint) error {
+	override := &models.PriceOverride{
+		LotName:   lotName,
+		Price:     price,
+		ValidFrom: time.Now(),
+		Reason:    reason,
+		CreatedBy: userID,
+	}
+	return s.priceRepo.CreatePriceOverride(override)
+}
+
+// UpdatePriceMethod changes the pricing method for a component
+func (s *Service) UpdatePriceMethod(lotName string, method models.PriceMethod, periodDays int) error {
+	component, err := s.componentRepo.GetByLotName(lotName)
+	if err != nil {
+		return err
+	}
+
+	component.PriceMethod = method
+	if periodDays > 0 {
+		component.PricePeriodDays = periodDays
+	}
+
+	if err := s.componentRepo.Update(component); err != nil {
+		return err
+	}
+
+	return s.UpdateComponentPrice(lotName)
+}
+
+// GetPriceStats returns statistics for a component's price history
+func (s *Service) GetPriceStats(lotName string, periodDays int) (*PriceStats, error) {
+	if periodDays == 0 {
+		periodDays = s.config.DefaultPeriodDays
+	}
+
+	points, err := s.priceRepo.GetPriceHistory(lotName, periodDays)
+	if err != nil {
+		return nil, err
+	}
+
+	if len(points) == 0 {
+		return &PriceStats{QuoteCount: 0}, nil
+	}
+
+	prices := make([]float64, len(points))
+	for i, p := range points {
+		prices[i] = p.Price
+	}
+
+	return &PriceStats{
+		QuoteCount:    len(points),
+		MinPrice:      CalculatePercentile(prices, 0),
+		MaxPrice:      CalculatePercentile(prices, 100),
+		MedianPrice:   CalculateMedian(prices),
+		AveragePrice:  CalculateAverage(prices),
+		StdDeviation:  CalculateStdDev(prices),
+		LatestPrice:   points[0].Price,
+		LatestDate:    points[0].Date,
+		OldestDate:    points[len(points)-1].Date,
+		Percentile25:  CalculatePercentile(prices, 25),
+		Percentile75:  CalculatePercentile(prices, 75),
+	}, nil
+}
+
+type PriceStats struct {
+	QuoteCount   int       `json:"quote_count"`
+	MinPrice     float64   `json:"min_price"`
+	MaxPrice     float64   `json:"max_price"`
+	MedianPrice  float64   `json:"median_price"`
+	AveragePrice float64   `json:"average_price"`
+	StdDeviation float64   `json:"std_deviation"`
+	LatestPrice  float64   `json:"latest_price"`
+	LatestDate   time.Time `json:"latest_date"`
+	OldestDate   time.Time `json:"oldest_date"`
+	Percentile25 float64   `json:"percentile_25"`
+	Percentile75 float64   `json:"percentile_75"`
+}