fix: resolve all golangci-lint errors and integrate linter into CI/CD, Makefile, and check script

internal/api/handlers.go

@@ -345,7 +345,9 @@ func (h *Handler) ImportOPML(w http.ResponseWriter, r *http.Request) {
 		h.writeError(w, http.StatusBadRequest, "file field is required")
 		return
 	}
-	defer file.Close()
+	defer func() {
+		_ = file.Close()
+	}()
 
 	data, err := io.ReadAll(file)
 	if err != nil {
@@ -524,7 +526,3 @@ func (h *Handler) ViewSharedEntry(w http.ResponseWriter, r *http.Request) {
 	w.Header().Set("Content-Type", "text/html; charset=utf-8")
 	_, _ = w.Write([]byte(html))
 }
-
-type sqlNullHelper struct {
-	String string
-}