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/quote.go

@@ -0,0 +1,139 @@
+package services
+
+import (
+	"errors"
+
+	"github.com/mchus/quoteforge/internal/models"
+	"github.com/mchus/quoteforge/internal/repository"
+	"github.com/mchus/quoteforge/internal/services/pricing"
+)
+
+var (
+	ErrEmptyQuote       = errors.New("quote cannot be empty")
+	ErrComponentNotFound = errors.New("component not found")
+	ErrNoPriceAvailable  = errors.New("no price available for component")
+)
+
+type QuoteService struct {
+	componentRepo *repository.ComponentRepository
+	statsRepo     *repository.StatsRepository
+	pricingService *pricing.Service
+}
+
+func NewQuoteService(
+	componentRepo *repository.ComponentRepository,
+	statsRepo *repository.StatsRepository,
+	pricingService *pricing.Service,
+) *QuoteService {
+	return &QuoteService{
+		componentRepo:  componentRepo,
+		statsRepo:      statsRepo,
+		pricingService: pricingService,
+	}
+}
+
+type QuoteItem struct {
+	LotName     string  `json:"lot_name"`
+	Quantity    int     `json:"quantity"`
+	UnitPrice   float64 `json:"unit_price"`
+	TotalPrice  float64 `json:"total_price"`
+	Description string  `json:"description"`
+	Category    string  `json:"category"`
+	HasPrice    bool    `json:"has_price"`
+}
+
+type QuoteValidationResult struct {
+	Valid    bool          `json:"valid"`
+	Items    []QuoteItem   `json:"items"`
+	Errors   []string      `json:"errors"`
+	Warnings []string      `json:"warnings"`
+	Total    float64       `json:"total"`
+}
+
+type QuoteRequest struct {
+	Items []struct {
+		LotName  string `json:"lot_name"`
+		Quantity int    `json:"quantity"`
+	} `json:"items"`
+}
+
+func (s *QuoteService) ValidateAndCalculate(req *QuoteRequest) (*QuoteValidationResult, error) {
+	if len(req.Items) == 0 {
+		return nil, ErrEmptyQuote
+	}
+
+	result := &QuoteValidationResult{
+		Valid:    true,
+		Items:    make([]QuoteItem, 0, len(req.Items)),
+		Errors:   make([]string, 0),
+		Warnings: make([]string, 0),
+	}
+
+	lotNames := make([]string, len(req.Items))
+	quantities := make(map[string]int)
+	for i, item := range req.Items {
+		lotNames[i] = item.LotName
+		quantities[item.LotName] = item.Quantity
+	}
+
+	components, err := s.componentRepo.GetMultiple(lotNames)
+	if err != nil {
+		return nil, err
+	}
+
+	componentMap := make(map[string]*models.LotMetadata)
+	for i := range components {
+		componentMap[components[i].LotName] = &components[i]
+	}
+
+	var total float64
+
+	for _, reqItem := range req.Items {
+		comp, exists := componentMap[reqItem.LotName]
+		if !exists {
+			result.Valid = false
+			result.Errors = append(result.Errors, "Component not found: "+reqItem.LotName)
+			continue
+		}
+
+		item := QuoteItem{
+			LotName:  reqItem.LotName,
+			Quantity: reqItem.Quantity,
+			HasPrice: false,
+		}
+
+		if comp.Lot != nil {
+			item.Description = comp.Lot.LotDescription
+		}
+		if comp.Category != nil {
+			item.Category = comp.Category.Code
+		}
+
+		// Get effective price (override or calculated)
+		price, err := s.pricingService.GetEffectivePrice(reqItem.LotName)
+		if err == nil && price != nil && *price > 0 {
+			item.UnitPrice = *price
+			item.TotalPrice = *price * float64(reqItem.Quantity)
+			item.HasPrice = true
+			total += item.TotalPrice
+		} else {
+			result.Warnings = append(result.Warnings, "No price available for: "+reqItem.LotName)
+		}
+
+		result.Items = append(result.Items, item)
+	}
+
+	result.Total = total
+	return result, nil
+}
+
+// RecordUsage records that components were used in a quote
+func (s *QuoteService) RecordUsage(items []models.ConfigItem) error {
+	for _, item := range items {
+		revenue := item.UnitPrice * float64(item.Quantity)
+		if err := s.statsRepo.IncrementUsage(item.LotName, item.Quantity, revenue); err != nil {
+			return err
+		}
+	}
+	return nil
+}