#220 feat: participant scopes over an OR group of columns

[RAprogramm]

Closes #220

"Rows where this principal takes part in any role" is an OR over several columns of the same type, and it was the one query shape that forced raw SQL. It is now a declaration:

#[scope(involving: requester_id | subject_id)]
#[scope(handled: requester_id | subject_id, within = parcel_id)]

Generated	SQL
list_involving(value, limit, offset)	WHERE (requester_id = $1 OR subject_id = $1) ORDER BY id DESC LIMIT $2 OFFSET $3
list_handled(parcel_id, value, limit, offset)	WHERE parcel_id = $1 AND (requester_id = $2 OR subject_id = $2) …

The value is bound once and referenced by every branch. within narrows the group to one parent before the OR. Soft-delete aware, ordered like list.

Checked while parsing: a scope needs at least two columns, every name must be a field of the entity, and the OR-ed columns must agree on their type — one value is bound against all of them, so a mismatch would otherwise surface as a decode error at run time. Two compile-fail cases cover the rejections.

A live-Postgres case proves both forms: both roles match the same principal, narrowing drops the row belonging to another parent, an uninvolved principal matches nothing, and pagination applies. README and the attribute page in all five wiki languages document it.

[codecov]

Codecov Report

❌ Patch coverage is 97.32143% with 6 lines in your changes missing coverage. Please review.
✅ All tests successful. No failed tests found.

Files with missing lines	Patch %	Lines
...derive-impl/src/entity/parse/entity/constructor.rs	87.50%	5 Missing ⚠️
...tity-derive-impl/src/entity/sql/postgres/scopes.rs	98.36%	1 Missing ⚠️

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