Make `@field()` mandatory for database fields?

[targos]

We're not sure we actually want this, but the current behavior (which forwards all non-special get/set operations to the $attributes) can be risky:

• It's possible to add unwanted fields to the documents by mistake (when any is involved)
• If a user wants to use internal class fields, they have to use #private JS fields or Symbols, otherwise save() will persist them to the database.