refactor: stop reading joinColumnName from relation field settings (#20304)

## Summary

`joinColumnName` on relation field settings is always derivable from the
field name (and the target object name for morph relations). This PR
stops reading it from settings anywhere in production code; the stored
value is no longer used.

The settings field is **not** removed from data yet — a follow-up can
drop it once we are confident nothing depends on the stored value.

## Helpers

The helpers are split by layer because frontend and backend hold morph
relations differently: the frontend has a base name plus a
`morphRelations[]` array, the backend has one row per target with the
name already morph-resolved.

| Helper | Layer | When to use |
|---|---|---|
| `computeRelationGqlFieldJoinColumnName` | Shared / frontend
(`gqlField`) | Non-morph relation on the frontend. |
| `computeMorphRelationGqlFieldName` | Shared / frontend (`gqlField`) |
Need the per-target morph gqlField name (e.g. `targetCompany`). |
| `computeMorphRelationGqlFieldJoinColumnName` | Shared / frontend
(`gqlField`) | Per-target morph join column on the frontend. Prefer over
the non-morph helper for any morph field — it forces the per-target
inputs. |
| `computeMorphOrRelationFieldJoinColumnName` | Backend
(`FlatFieldMetadata.name`) | Any backend read or write — the flat name
is already morph-resolved, so one helper covers both cases. |
| `computeMorphRelationFlatFieldName` | Backend
(`FlatFieldMetadata.name`) | **Mutation paths only** (create / update /
object rename). Reads consume the stored `field.name` and never call
this. |

## Test plan

- [x] Typecheck and lint (front, server, shared)
- [x] Existing unit tests pass
- [ ] CI green

packages/twenty-server/src/engine/api/common/common-select-fields/utils/get-all-selectable-fields.util.ts

@@ -9,8 +9,8 @@ import {
   compositeTypeDefinitions,
 } from 'twenty-shared/types';
 
-import { isCompositeFieldMetadataType } from 'src/engine/metadata-modules/field-metadata/utils/is-composite-field-metadata-type.util';
 import { computeMorphOrRelationFieldJoinColumnName } from 'src/engine/metadata-modules/field-metadata/utils/compute-morph-or-relation-field-join-column-name.util';
+import { isCompositeFieldMetadataType } from 'src/engine/metadata-modules/field-metadata/utils/is-composite-field-metadata-type.util';
 import { type FlatEntityMaps } from 'src/engine/metadata-modules/flat-entity/types/flat-entity-maps.type';
 import { findFlatEntityByIdInFlatEntityMapsOrThrow } from 'src/engine/metadata-modules/flat-entity/utils/find-flat-entity-by-id-in-flat-entity-maps-or-throw.util';
 import { type FlatFieldMetadata } from 'src/engine/metadata-modules/flat-field-metadata/types/flat-field-metadata.type';

packages/twenty-server/src/engine/api/common/common-select-fields/utils/get-is-flat-field-a-junction-relation-field.ts

@@ -11,8 +11,7 @@ export const getIsFlatFieldAJunctionRelationField = ({
   const isJunctionRelationField =
     isDefined(flatField.settings) &&
     'relationType' in flatField.settings &&
-    flatField.settings.relationType === RelationType.MANY_TO_ONE &&
-    isDefined(flatField.settings.joinColumnName);
+    flatField.settings.relationType === RelationType.MANY_TO_ONE;
 
   // TODO: refactor this when we remove hard-coded activity relations
   const isActivityRelationField =