AddLink inserts a new URL into the database.

AddUser inserts a new user into the database.

DeleteLink deletes a URL from the database based on its short URL.

GetLink retrieves a URL from the database based on its long URL.

GetLinkByShortURL retrieves a URL from the database based on its short URL.

GetUserByUsername retrieves a user from the database based on their username and password.

IncrementUsage increments the usage count for a URL in the database.