#219 feat: chainable setters on update DTOs

[RAprogramm]

Closes #219

A patch built from a struct literal has to spell the wrapping out — Some(value) for a plain column, Some(Some(value)) for a nullable one, Some(None) to write NULL. Correct, and opaque at the call site.

Update DTOs now carry setters that say it in the caller's words:

Generated	For
set_{field}(value)	every updatable column
clear_{field}()	nullable columns — asks for NULL, which is not the same as leaving the field alone
expecting_version(v)	entities declaring #[version]

let patch = UpdateParcelRequest::default()
    .set_status("accepted".into())
    .set_courier_id(courier)
    .expecting_version(3);

Purely additive: struct-literal construction is untouched, and the serialized shape does not change.

Covered by a compile-pass case for the produced values and a live-Postgres case proving the built patch reaches the row — including that clear_ writes NULL while an untouched field keeps its stored value. README and the attribute page in all five wiki languages document it.

[codecov]

Codecov Report

❌ Patch coverage is 96.96970% with 1 line in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
crates/entity-derive-impl/src/entity/dto.rs	96.96%	1 Missing ⚠️

📢 Thoughts on this report? Let us know!