feat: implement Modrinth modpack importer (.mrpack)

internal/api/modpack_handler.go

@@ -46,6 +46,8 @@ func (h *ModpackHandler) ImportModpack(w http.ResponseWriter, r *http.Request) {
 			return
 		}
 		imp = importer.NewCurseForgeImporter(storagePath, apiKey)
+	case "modrinth":
+		imp = importer.NewModrinthImporter(storagePath)
 	default:
 		http.Error(w, "Invalid importer type", http.StatusBadRequest)
 		return