CUB-113: implement core CRUD API endpoints

- Add dtos package with request/response structs
- Add repositories: Material, Filament, Printer, PrintJob, UsageLog
- Add services: FilamentService, PrinterService, PrintJobService
- Add handlers for all 5 resources with consistent error responses
- Wire all endpoints into Chi router under /api
- Validation on POST/PUT filament endpoints
- Filter/pagination support on list endpoints
- Soft-delete for filaments (DELETE /api/filaments/{id})
- go build ./... && go vet ./... → PASS

backend/internal/handlers/print_job_handler.go

@@ -0,0 +1,60 @@
+package handlers
+
+import (
+	"log/slog"
+	"net/http"
+	"strconv"
+
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/dtos"
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/repositories"
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/services"
+)
+
+// PrintJobHandler handles HTTP requests for print job operations.
+type PrintJobHandler struct {
+	service *services.PrintJobService
+}
+
+// NewPrintJobHandler creates a PrintJobHandler with the given service.
+func NewPrintJobHandler(service *services.PrintJobService) *PrintJobHandler {
+	return &PrintJobHandler{service: service}
+}
+
+// List handles GET /api/print-jobs — returns paginated, filtered print jobs.
+func (h *PrintJobHandler) List(w http.ResponseWriter, r *http.Request) {
+	limit, offset := parsePagination(r)
+	filter := repositories.PrintJobFilter{
+		Status: r.URL.Query().Get("status"),
+		Limit:  limit,
+		Offset: offset,
+	}
+
+	if pidStr := r.URL.Query().Get("printer_id"); pidStr != "" {
+		pid, err := strconv.Atoi(pidStr)
+		if err != nil {
+			writeJSON(w, http.StatusBadRequest, dtos.ErrorResponse{
+				Error: "invalid printer_id",
+				Code:  http.StatusBadRequest,
+			})
+			return
+		}
+		filter.PrinterID = &pid
+	}
+
+	jobs, total, err := h.service.List(r.Context(), filter)
+	if err != nil {
+		slog.Error("failed to list print jobs", "error", err)
+		writeJSON(w, http.StatusInternalServerError, dtos.ErrorResponse{
+			Error: "internal server error",
+			Code:  http.StatusInternalServerError,
+		})
+		return
+	}
+
+	writeJSON(w, http.StatusOK, dtos.ListResponse{
+		Data:   jobs,
+		Total:  total,
+		Limit:  limit,
+		Offset: offset,
+	})
+}