Add input validation

[JakenHerman]

We need to modify the function add_todo in src/routes.rs to validate input a little bit before processing it. (e.g., checking if the title is not empty). If the input is not valid, we should not add it to the database. Similarly, we shouldn't allow already completed tasks to be added (new to-dos should have the completed field set to false initially.

Let's make this modification and update tests in tests/add_todo.rs accordingly. We should have a success test case as well as an invalid test case.