update entity properties, getters, and setters to be strongly typed; use __construct to ensure entities are newed up in a valid state

**context:** the `make:entity` is pretty good for creating and updating entities, but it allows many properties to be `null`, even if they shouldn't be. this can lead to bugs.

to reduce the chance of bugs, aim for objects to always be in a valid state.

**todo:**
1. update entity properties, and related getter and setter, to be strongly-typed (including accounting for nullability)
2. ensure entities are always newed up in a valid state; possibilities:
1. add required properties to `__construct` - easy, works best when required properties are low
* this option will probably cover the majority of cases
3. make `__construct` private and create static factory methods - similar to above; preferable when there are multiple ways to construct an entity
4. create a builder class (preferably with a fluent API) that assists with creating the entity - appropriate only when newing up an entity is very complex

it's totally legit to upgrade just one entity or a small group of entities in a PR; this issue will be closed when all entities are updated.

Start by reading the make:entity implementation and inspecting a generated entity, including its properties, getters, setters, and construction path. Define a manageable entity or small group to update first; done means its types account for nullability and it can only be created in a valid state, with the chosen construction approach documented by the resulting code.

Written by the indexing model from the issue text.