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/calculator.go

@@ -0,0 +1,121 @@
+package pricing
+
+import (
+	"math"
+	"sort"
+	"time"
+
+	"github.com/mchus/quoteforge/internal/repository"
+)
+
+// CalculateMedian returns the median of prices
+func CalculateMedian(prices []float64) float64 {
+	if len(prices) == 0 {
+		return 0
+	}
+
+	sorted := make([]float64, len(prices))
+	copy(sorted, prices)
+	sort.Float64s(sorted)
+
+	n := len(sorted)
+	if n%2 == 0 {
+		return (sorted[n/2-1] + sorted[n/2]) / 2
+	}
+	return sorted[n/2]
+}
+
+// CalculateAverage returns the arithmetic mean of prices
+func CalculateAverage(prices []float64) float64 {
+	if len(prices) == 0 {
+		return 0
+	}
+
+	var sum float64
+	for _, p := range prices {
+		sum += p
+	}
+	return sum / float64(len(prices))
+}
+
+// CalculateWeightedMedian calculates median with exponential decay weights
+// More recent prices have higher weight
+func CalculateWeightedMedian(points []repository.PricePoint, decayDays int) float64 {
+	if len(points) == 0 {
+		return 0
+	}
+
+	type weightedPrice struct {
+		price  float64
+		weight float64
+	}
+
+	now := time.Now()
+	weighted := make([]weightedPrice, len(points))
+	var totalWeight float64
+
+	for i, p := range points {
+		daysSince := now.Sub(p.Date).Hours() / 24
+		// weight = e^(-days / decay_days)
+		weight := math.Exp(-daysSince / float64(decayDays))
+		weighted[i] = weightedPrice{price: p.Price, weight: weight}
+		totalWeight += weight
+	}
+
+	// Sort by price
+	sort.Slice(weighted, func(i, j int) bool {
+		return weighted[i].price < weighted[j].price
+	})
+
+	// Find weighted median
+	targetWeight := totalWeight / 2
+	var cumulativeWeight float64
+
+	for _, wp := range weighted {
+		cumulativeWeight += wp.weight
+		if cumulativeWeight >= targetWeight {
+			return wp.price
+		}
+	}
+
+	return weighted[len(weighted)-1].price
+}
+
+// CalculatePercentile calculates the nth percentile of prices
+func CalculatePercentile(prices []float64, percentile float64) float64 {
+	if len(prices) == 0 {
+		return 0
+	}
+
+	sorted := make([]float64, len(prices))
+	copy(sorted, prices)
+	sort.Float64s(sorted)
+
+	index := (percentile / 100) * float64(len(sorted)-1)
+	lower := int(math.Floor(index))
+	upper := int(math.Ceil(index))
+
+	if lower == upper {
+		return sorted[lower]
+	}
+
+	fraction := index - float64(lower)
+	return sorted[lower]*(1-fraction) + sorted[upper]*fraction
+}
+
+// CalculateStdDev calculates standard deviation
+func CalculateStdDev(prices []float64) float64 {
+	if len(prices) < 2 {
+		return 0
+	}
+
+	mean := CalculateAverage(prices)
+	var sumSquares float64
+
+	for _, p := range prices {
+		diff := p - mean
+		sumSquares += diff * diff
+	}
+
+	return math.Sqrt(sumSquares / float64(len(prices)-1))
+}