Replace databaseInterface with Django models

~~`databaseInterface` is still imported in a few places in MCPServer, primarily for the `getUTCTime` function that will be moved elsewhere, and for database locking that may be removeable.~~

See comments below.
I think the old code checked for existence plus start time being non-zero; the logging below indicates it primarily cares about the latter (since the task should always exist here). The implicit assumption appeared to be that there would always be a Task with the requested taskUUID.