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/activities/utils/getActivityTargetObjectRecords.ts

@@ -10,7 +10,10 @@ import {
   FieldMetadataType,
   type Nullable,
 } from 'twenty-shared/types';
-import { computeMorphRelationFieldName, isDefined } from 'twenty-shared/utils';
+import {
+  computeMorphRelationGqlFieldName,
+  isDefined,
+} from 'twenty-shared/utils';
 
 type GetActivityTargetObjectRecordsProps = {
   activityRecord: Note | Task;
@@ -94,7 +97,7 @@ export const getActivityTargetObjectRecords = ({
         ) {
           const matchingMorphRelation = field.morphRelations.find(
             (morphRelation) => {
-              const morphFieldName = computeMorphRelationFieldName({
+              const morphFieldName = computeMorphRelationGqlFieldName({
                 fieldName: field.name,
                 relationType: morphRelation.type,
                 targetObjectMetadataNameSingular:
@@ -108,7 +111,7 @@ export const getActivityTargetObjectRecords = ({
           );
 
           if (isDefined(matchingMorphRelation)) {
-            matchingFieldName = computeMorphRelationFieldName({
+            matchingFieldName = computeMorphRelationGqlFieldName({
               fieldName: field.name,
               relationType: matchingMorphRelation.type,
               targetObjectMetadataNameSingular: