### Scope
Models A to D
### What
Refactor OAS for models to accurately represent their shape in API responses.
### Why
About 33% of model fields are not accurately represented in the OAS. Most of the issues are:
- fields that can not be omitted in the response are not declared as `required`
- fields that could return `null` as their value are not declared as `nullable: true`
When using a code generator, these OAS issues would lead to inaccurate response shapes in the generated client.
### How
#### nullable
Fields meeting at least one of the following condition will be represented as `nullable: true` in OAS:
* The field is decorated with `@Column({ nullable: true })`
* The field is decorated with `@OneToOne`, `@ManyToOne`
* The field is decorated with `@DeleteDateColumn`
#### optional
Fields meeting at least one of the following conditions will never be listed as `required` in OAS and will be considered optional and could be omitted in the response:
* The field is decorated with `@OneToOne`, `@ManyToOne`, `@OneToMany`, `@ManyToMany`
* The field is decorated with `@FeatureFlagColumn`
* The field is decorated with `@Column({select: false})`
* The field is representing dynamic values not persisted in the database
Fields not meeting any of the conditions above will be declared as `required` and are expected to be present in the response.
### Test
* Ran OAS validator.
* Ran docs build script.
Expect OAS changes to be reflected in the API documentation.
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