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/metadata-modules/index-metadata/utils/generate-flat-index.util.ts

@@ -1,9 +1,11 @@
+import { RelationType } from 'twenty-shared/types';
 import { isDefined } from 'twenty-shared/utils';
 
 import {
   FlatEntityMapsException,
   FlatEntityMapsExceptionCode,
 } from 'src/engine/metadata-modules/flat-entity/exceptions/flat-entity-maps.exception';
+import { computeMorphOrRelationFieldJoinColumnName } from 'src/engine/metadata-modules/field-metadata/utils/compute-morph-or-relation-field-join-column-name.util';
 import { isMorphOrRelationUniversalFlatFieldMetadata } from 'src/engine/metadata-modules/flat-field-metadata/utils/is-morph-or-relation-flat-field-metadata.util';
 import { generateDeterministicIndexNameV2 } from 'src/engine/metadata-modules/index-metadata/utils/generate-deterministic-index-name-v2';
 import { type UniversalFlatFieldMetadata } from 'src/engine/workspace-manager/workspace-migration/universal-flat-entity/types/universal-flat-field-metadata.type';
@@ -36,11 +38,15 @@ export const generateFlatIndexMetadataWithNameOrThrow = ({
         );
       }
 
-      const name = isMorphOrRelationUniversalFlatFieldMetadata(
-        relatedFlatFieldMetadata,
-      )
-        ? (relatedFlatFieldMetadata.universalSettings.joinColumnName ??
-          relatedFlatFieldMetadata.name)
+      const isManyToOneRelation =
+        isMorphOrRelationUniversalFlatFieldMetadata(relatedFlatFieldMetadata) &&
+        relatedFlatFieldMetadata.universalSettings?.relationType ===
+          RelationType.MANY_TO_ONE;
+
+      const name = isManyToOneRelation
+        ? computeMorphOrRelationFieldJoinColumnName({
+            name: relatedFlatFieldMetadata.name,
+          })
         : relatedFlatFieldMetadata.name;
 
       return {