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-front/src/modules/object-record/graphql/record-gql-fields/utils/generateDepthRecordGqlFieldsFromFields.ts

@@ -11,7 +11,10 @@ import { buildIdentifierGqlFields } from '@/object-record/graphql/record-gql-fie
 import { generateActivityTargetGqlFields } from '@/object-record/graphql/record-gql-fields/utils/generateActivityTargetGqlFields';
 import { generateJunctionRelationGqlFields } from '@/object-record/graphql/record-gql-fields/utils/generateJunctionRelationGqlFields';
 import { isJunctionRelationField } from '@/object-record/record-field/ui/utils/junction/isJunctionRelationField';
-import { computeMorphRelationFieldName, isDefined } from 'twenty-shared/utils';
+import {
+  computeMorphRelationGqlFieldName,
+  isDefined,
+} from 'twenty-shared/utils';
 
 export type GenerateDepthRecordGqlFieldsFromFields = {
   objectMetadataItems: Pick<
@@ -138,7 +141,7 @@ export const generateDepthRecordGqlFieldsFromFields = ({
             }
 
             return {
-              gqlField: computeMorphRelationFieldName({
+              gqlField: computeMorphRelationGqlFieldName({
                 fieldName: fieldMetadata.name,
                 relationType: morphRelation.type,
                 targetObjectMetadataNameSingular:

packages/twenty-front/src/modules/object-record/graphql/record-gql-fields/utils/generateJunctionRelationGqlFields.ts

@@ -6,7 +6,10 @@ import {
   type JunctionObjectMetadataItem,
 } from '@/object-record/record-field/ui/utils/junction/getJunctionConfig';
 import { FieldMetadataType } from 'twenty-shared/types';
-import { computeMorphRelationFieldName, isDefined } from 'twenty-shared/utils';
+import {
+  computeMorphRelationGqlFieldName,
+  isDefined,
+} from 'twenty-shared/utils';
 
 type JunctionFieldMetadataItem = Pick<
   FieldMetadataItem,
@@ -56,7 +59,7 @@ const buildMorphTargetFieldGqlFields = (
       continue;
     }
 
-    const computedFieldName = computeMorphRelationFieldName({
+    const computedFieldName = computeMorphRelationGqlFieldName({
       fieldName: morphRelation.sourceFieldMetadata.name,
       relationType: morphRelation.type,
       targetObjectMetadataNameSingular: targetObjectMetadata.nameSingular,