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/repository/price.go

@@ -0,0 +1,99 @@
+package repository
+
+import (
+	"time"
+
+	"github.com/mchus/quoteforge/internal/models"
+	"gorm.io/gorm"
+)
+
+type PriceRepository struct {
+	db *gorm.DB
+}
+
+func NewPriceRepository(db *gorm.DB) *PriceRepository {
+	return &PriceRepository{db: db}
+}
+
+type PricePoint struct {
+	Price float64
+	Date  time.Time
+}
+
+// GetPriceHistory returns price history from lot_log for a component
+func (r *PriceRepository) GetPriceHistory(lotName string, periodDays int) ([]PricePoint, error) {
+	var points []PricePoint
+	since := time.Now().AddDate(0, 0, -periodDays)
+
+	err := r.db.Model(&models.LotLog{}).
+		Select("price, date").
+		Where("lot = ? AND date >= ?", lotName, since).
+		Order("date DESC").
+		Scan(&points).Error
+
+	return points, err
+}
+
+// GetLatestPrice returns the most recent price for a component
+func (r *PriceRepository) GetLatestPrice(lotName string) (*PricePoint, error) {
+	var point PricePoint
+	err := r.db.Model(&models.LotLog{}).
+		Select("price, date").
+		Where("lot = ?", lotName).
+		Order("date DESC").
+		First(&point).Error
+	if err != nil {
+		return nil, err
+	}
+	return &point, nil
+}
+
+// GetPriceOverride returns active override for a component
+func (r *PriceRepository) GetPriceOverride(lotName string) (*models.PriceOverride, error) {
+	var override models.PriceOverride
+	today := time.Now().Truncate(24 * time.Hour)
+
+	err := r.db.
+		Where("lot_name = ?", lotName).
+		Where("valid_from <= ?", today).
+		Where("valid_until IS NULL OR valid_until >= ?", today).
+		Order("valid_from DESC").
+		First(&override).Error
+
+	if err != nil {
+		return nil, err
+	}
+	return &override, nil
+}
+
+// CreatePriceOverride creates a new price override
+func (r *PriceRepository) CreatePriceOverride(override *models.PriceOverride) error {
+	return r.db.Create(override).Error
+}
+
+// GetPriceOverrides returns all overrides for a component
+func (r *PriceRepository) GetPriceOverrides(lotName string) ([]models.PriceOverride, error) {
+	var overrides []models.PriceOverride
+	err := r.db.
+		Where("lot_name = ?", lotName).
+		Order("valid_from DESC").
+		Find(&overrides).Error
+	return overrides, err
+}
+
+// DeletePriceOverride deletes an override
+func (r *PriceRepository) DeletePriceOverride(id uint) error {
+	return r.db.Delete(&models.PriceOverride{}, id).Error
+}
+
+// GetQuoteCount returns the number of quotes in lot_log for a period
+func (r *PriceRepository) GetQuoteCount(lotName string, periodDays int) (int64, error) {
+	var count int64
+	since := time.Now().AddDate(0, 0, -periodDays)
+
+	err := r.db.Model(&models.LotLog{}).
+		Where("lot = ? AND date >= ?", lotName, since).
+		Count(&count).Error
+
+	return count, err
+}