Investigate restructuring of backend Notion service to comply with DI pattern

[jdtech3]

I see, we can revisit this at a later point to find a way to structure this code so it complies with DI without needing to duplicate code. Closing these as resolved

Originally posted by @DM1122 in #9 (comment)

- const _getNotionDB = async function(database_id, notion_client = undefined) {
-  if (notion_client === undefined) notion_client = newNotionClient();

+ const _getNotionDB = async function(database_id, notion_client) {

(server/src/services/notion.service.js)