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/field-metadata/utils/compute-morph-or-relation-field-join-column-name.util.ts

@@ -1,9 +1,9 @@
-type ComputeRelationFieldJoinColumnNameArgs = {
+type ComputeMorphOrRelationFieldJoinColumnNameArgs = {
   name: string;
 };
 
 export const computeMorphOrRelationFieldJoinColumnName = ({
   name,
-}: ComputeRelationFieldJoinColumnNameArgs) => {
+}: ComputeMorphOrRelationFieldJoinColumnNameArgs): string => {
   return `${name}Id`;
 };

packages/twenty-server/src/engine/metadata-modules/field-metadata/utils/compute-morph-relation-flat-field-name.util.ts

@@ -0,0 +1,34 @@
+import { RelationType } from 'twenty-shared/types';
+import { capitalize } from 'twenty-shared/utils';
+
+import {
+  FieldMetadataException,
+  FieldMetadataExceptionCode,
+} from 'src/engine/metadata-modules/field-metadata/field-metadata.exception';
+
+type ComputeMorphRelationFlatFieldNameArgs = {
+  fieldName: string;
+  relationType: RelationType;
+  targetObjectMetadataNameSingular: string;
+  targetObjectMetadataNamePlural: string;
+};
+
+export const computeMorphRelationFlatFieldName = ({
+  fieldName,
+  relationType,
+  targetObjectMetadataNameSingular: nameSingular,
+  targetObjectMetadataNamePlural: namePlural,
+}: ComputeMorphRelationFlatFieldNameArgs): string => {
+  if (relationType === RelationType.MANY_TO_ONE) {
+    return `${fieldName}${capitalize(nameSingular)}`;
+  }
+
+  if (relationType === RelationType.ONE_TO_MANY) {
+    return `${fieldName}${capitalize(namePlural)}`;
+  }
+
+  throw new FieldMetadataException(
+    `Invalid relation type (${relationType}) for field ${fieldName} on ${nameSingular}`,
+    FieldMetadataExceptionCode.INVALID_FIELD_INPUT,
+  );
+};