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/printer_handler.go

@@ -0,0 +1,34 @@
+package handlers
+
+import (
+	"log/slog"
+	"net/http"
+
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/dtos"
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/services"
+)
+
+// PrinterHandler handles HTTP requests for printer listings.
+type PrinterHandler struct {
+	service *services.PrinterService
+}
+
+// NewPrinterHandler creates a PrinterHandler with the given service.
+func NewPrinterHandler(service *services.PrinterService) *PrinterHandler {
+	return &PrinterHandler{service: service}
+}
+
+// List handles GET /api/printers — returns all printers with printer_type info.
+func (h *PrinterHandler) List(w http.ResponseWriter, r *http.Request) {
+	printers, err := h.service.List(r.Context())
+	if err != nil {
+		slog.Error("failed to list printers", "error", err)
+		writeJSON(w, http.StatusInternalServerError, dtos.ErrorResponse{
+			Error: "internal server error",
+			Code:  http.StatusInternalServerError,
+		})
+		return
+	}
+
+	writeJSON(w, http.StatusOK, dtos.SingleResponse{Data: printers})
+}