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/apollo/optimistic-effect/utils/triggerUpdateRelationsOptimisticEffect.ts

@@ -20,7 +20,7 @@ import { type ObjectRecord } from '@/object-record/types/ObjectRecord';
 import { type ApolloCache } from '@apollo/client';
 import { isArray } from '@sniptt/guards';
 import {
-  computeMorphRelationFieldName,
+  computeMorphRelationGqlFieldName,
   CustomError,
   isDefined,
 } from 'twenty-shared/utils';
@@ -186,7 +186,7 @@ const triggerUpdateRelationOptimisticEffect = ({
   const gqlFieldNameOnTargetRecord =
     targetFieldMetadataFullObject.type === FieldMetadataType.RELATION
       ? targetFieldMetadataFullObject.name
-      : computeMorphRelationFieldName({
+      : computeMorphRelationGqlFieldName({
           fieldName: targetFieldMetadataFullObject.name,
           relationType: targetFieldMetadataFullObject.settings?.relationType,
           targetObjectMetadataNameSingular:
@@ -276,7 +276,7 @@ const triggerUpdateMorphRelationOptimisticEffect = ({
   }
 
   morphRelations.forEach((morphRelation) => {
-    const gqlFieldMorphRelation = computeMorphRelationFieldName({
+    const gqlFieldMorphRelation = computeMorphRelationGqlFieldName({
       fieldName: fieldMetadataItemOnSourceRecord.name,
       relationType: morphRelation.type,
       targetObjectMetadataNameSingular: