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/usage_log_handler.go

@@ -0,0 +1,70 @@
+package handlers
+
+import (
+	"log/slog"
+	"net/http"
+	"strconv"
+
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/dtos"
+	"github.com/CubeCraft-Creations/Extrudex/backend/internal/repositories"
+)
+
+// UsageLogHandler handles HTTP requests for usage log operations.
+type UsageLogHandler struct {
+	repo *repositories.UsageLogRepository
+}
+
+// NewUsageLogHandler creates a UsageLogHandler with the given repository.
+func NewUsageLogHandler(repo *repositories.UsageLogRepository) *UsageLogHandler {
+	return &UsageLogHandler{repo: repo}
+}
+
+// List handles GET /api/usage-logs — returns paginated, filtered usage logs.
+func (h *UsageLogHandler) List(w http.ResponseWriter, r *http.Request) {
+	limit, offset := parsePagination(r)
+	filter := repositories.UsageLogFilter{
+		Limit:  limit,
+		Offset: offset,
+	}
+
+	if sidStr := r.URL.Query().Get("spool_id"); sidStr != "" {
+		sid, err := strconv.Atoi(sidStr)
+		if err != nil {
+			writeJSON(w, http.StatusBadRequest, dtos.ErrorResponse{
+				Error: "invalid spool_id",
+				Code:  http.StatusBadRequest,
+			})
+			return
+		}
+		filter.SpoolID = &sid
+	}
+
+	if jidStr := r.URL.Query().Get("job_id"); jidStr != "" {
+		jid, err := strconv.Atoi(jidStr)
+		if err != nil {
+			writeJSON(w, http.StatusBadRequest, dtos.ErrorResponse{
+				Error: "invalid job_id",
+				Code:  http.StatusBadRequest,
+			})
+			return
+		}
+		filter.JobID = &jid
+	}
+
+	logs, total, err := h.repo.GetAll(r.Context(), filter)
+	if err != nil {
+		slog.Error("failed to list usage logs", "error", err)
+		writeJSON(w, http.StatusInternalServerError, dtos.ErrorResponse{
+			Error: "internal server error",
+			Code:  http.StatusInternalServerError,
+		})
+		return
+	}
+
+	writeJSON(w, http.StatusOK, dtos.ListResponse{
+		Data:   logs,
+		Total:  total,
+		Limit:  limit,
+		Offset: offset,
+	})
+}