## Summary
This PR enables sorting records by fields of related objects. For
example, sorting **People by their Company's name**.
### Before
Only scalar and composite fields could be sorted. Relation fields showed
in the sort dropdown but produced errors.
### After
Many-to-One relation fields can now be sorted using the related object's
**label identifier field** (e.g., Company's `name`).
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## Changes
### Frontend
- Added `RELATION` to sortable field types (restricted to `MANY_TO_ONE`
relations)
- New `getOrderByForRelationField()` generates nested orderBy structures
using the related object's label identifier
- Updated `turnSortsIntoOrderBy()` to handle relation fields by looking
up related object metadata
### Backend
- Extended `GraphqlQueryOrderFieldParser.parse()` to detect nested
relation ordering like `{ company: { name: 'AscNullsLast' } }`
- Returns `ParseOrderByResult` containing both `orderBy` conditions and
`relationJoins` info
- Added LEFT JOINs for relation ordering in `applyOrderToBuilder()`
- Added `addRelationOrderColumnsToBuilder()` for TypeORM DISTINCT
compatibility
### Tests
- Added unit tests for `filterSortableFieldMetadataItems`,
`getOrderByForRelationField`, and `turnSortsIntoOrderBy`
- Added integration tests covering ascending/descending order and
composite label identifiers
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## TypeORM Bug Workaround
We encountered a significant TypeORM limitation when implementing this
feature. When using `getMany()` with `ORDER BY` on joined relation
columns, TypeORM generates a DISTINCT subquery that has specific
requirements:
### Issue 1: Alias Parsing
TypeORM's `orderBy()` method fails with **"alias not found"** when using
quoted SQL identifiers like `"company"."name"`. TypeORM internally
expects unquoted property paths (e.g., `company.name`) for its alias
resolution mechanism.
### Issue 2: setFindOptions Clears addSelect
`setFindOptions({ select })` **clears any previously added `addSelect()`
columns**. This caused `"column distinctAlias.company_name does not
exist"` errors because the relation columns needed for ORDER BY were
being removed.
### Solution
We split the logic into two methods:
1. `applyOrderToBuilder()` - adds JOINs and ORDER BY (before
`setFindOptions`)
2. `addRelationOrderColumnsToBuilder()` - adds relation columns for
SELECT (AFTER `setFindOptions`)
This ensures the relation columns are present in the final SQL query's
SELECT clause with the proper underscore aliases (`company_name`) that
TypeORM's DISTINCT subquery expects.
**Related TypeORM issue**:
https://github.com/typeorm/typeorm/issues/9921
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## Screenshots/Demo
_Add screenshots if applicable_
