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/handlers/quote.go

@@ -0,0 +1,51 @@
+package handlers
+
+import (
+	"net/http"
+
+	"github.com/gin-gonic/gin"
+	"github.com/mchus/quoteforge/internal/services"
+)
+
+type QuoteHandler struct {
+	quoteService *services.QuoteService
+}
+
+func NewQuoteHandler(quoteService *services.QuoteService) *QuoteHandler {
+	return &QuoteHandler{quoteService: quoteService}
+}
+
+func (h *QuoteHandler) Validate(c *gin.Context) {
+	var req services.QuoteRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+
+	result, err := h.quoteService.ValidateAndCalculate(&req)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusOK, result)
+}
+
+func (h *QuoteHandler) Calculate(c *gin.Context) {
+	var req services.QuoteRequest
+	if err := c.ShouldBindJSON(&req); err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+
+	result, err := h.quoteService.ValidateAndCalculate(&req)
+	if err != nil {
+		c.JSON(http.StatusBadRequest, gin.H{"error": err.Error()})
+		return
+	}
+
+	c.JSON(http.StatusOK, gin.H{
+		"items": result.Items,
+		"total": result.Total,
+	})
+}