Add databases to the backend api's

The Speaker & Conferences API's now keep their state using an in-memory list.

It would be better to use something else for this. My suggestion is to use the Serverless offering of Cosmos DB for this to keep costs to a minimum.
Connecting to it should be done by using the managed identity of the App Service.

Start by examining the Speaker and Conferences APIs and their current in-memory state handling. Confirm how Azure Cosmos DB Serverless and the App Service managed identity should be used; done means both APIs persist state in the database instead of an in-memory list.

Written by the indexing model from the issue text.