fix: add indexes on timelineActivity FK columns for custom object morph relations

https://sonarly.com/issue/16678?type=bug

The `timelineActivity` table lacks database indexes on foreign key columns created for custom object morph relations, causing a 3,491ms sequential scan when loading a single custom object record's timeline activities.

Fix: **What changed:** When creating a custom object, Twenty automatically creates default relations to standard objects like `timelineActivity`, `noteTarget`, `taskTarget`, `attachment`, and `favorite`. The function `generateMorphOrRelationFlatFieldMetadataPair` already generates both the relation field metadata AND a BTREE index on the FK join column for the MANY_TO_ONE side. However, in `buildDefaultRelationFlatFieldMetadatasForCustomObject`, the destructuring `const { flatFieldMetadatas } = generateMorphOrRelationFlatFieldMetadataPair(...)` was silently discarding the `indexMetadatas` return value.

**The fix** (2 files, ~10 lines changed):

1. **`build-default-relation-flat-field-metadatas-for-custom-object.util.ts`**: Added `standardRelationIndexMetadatas` to the accumulator record type. Now destructures `indexMetadatas` alongside `flatFieldMetadatas` and accumulates them across all default relation objects.

2. **`from-create-object-input-to-flat-object-metadata-and-flat-field-metadatas-to-create.util.ts`**: Spreads the new `standardRelationIndexMetadatas` into `flatIndexMetadataToCreate` alongside the existing search vector GIN index.

**Why this is the right fix:** The index generation logic was already correct in `generateMorphOrRelationFlatFieldMetadataPair` (used successfully for user-created relation fields). The bug was purely that the custom object creation pipeline discarded the generated indexes. This was explicitly noted as a TODO in commit `fbd3286792` ("Index v2 side effects"): *"Will handle morph indexes in a new dedicated PR."*

**Note for existing workspaces:** This fix only affects NEW custom object creation. Existing workspaces with custom objects will still have missing indexes on their standard object tables. A one-time migration or `workspace:sync-metadata` run would be needed to backfill those indexes for existing custom objects.

packages/twenty-server/src/engine/metadata-modules/flat-object-metadata/utils/from-create-object-input-to-flat-object-metadata-and-flat-field-metadatas-to-create.util.ts

@@ -104,6 +104,7 @@ export const fromCreateObjectInputToFlatObjectMetadataAndFlatFieldMetadatasToCre
     const {
       standardSourceFlatFieldMetadatas,
       standardTargetFlatFieldMetadatas,
+      standardRelationIndexMetadatas,
     } = buildDefaultRelationFlatFieldMetadatasForCustomObject({
       existingFlatObjectMetadataMaps,
       sourceFlatObjectMetadata: universalFlatObjectMetadataToCreate,
@@ -124,9 +125,10 @@ export const fromCreateObjectInputToFlatObjectMetadataAndFlatFieldMetadatasToCre
 
     return {
       flatObjectMetadataToCreate: universalFlatObjectMetadataToCreate,
-      flatIndexMetadataToCreate: Object.values(
-        defaultIndexesForCustomObject.indexes,
-      ),
+      flatIndexMetadataToCreate: [
+        ...Object.values(defaultIndexesForCustomObject.indexes),
+        ...standardRelationIndexMetadatas,
+      ],
       relationTargetFlatFieldMetadataToCreate: standardTargetFlatFieldMetadatas,
       flatFieldMetadataToCreateOnObject: objectFlatFieldMetadatas,
     };

packages/twenty-server/src/engine/metadata-modules/object-metadata/utils/build-default-relation-flat-field-metadatas-for-custom-object.util.ts

@@ -20,6 +20,7 @@ import {
 } from 'src/engine/metadata-modules/object-metadata/object-metadata.exception';
 import { STANDARD_OBJECT_ICONS } from 'src/engine/workspace-manager/workspace-migration/constant/standard-object-icons';
 import { type UniversalFlatFieldMetadata } from 'src/engine/workspace-manager/workspace-migration/universal-flat-entity/types/universal-flat-field-metadata.type';
+import { type UniversalFlatIndexMetadata } from 'src/engine/workspace-manager/workspace-migration/universal-flat-entity/types/universal-flat-index-metadata.type';
 import { type UniversalFlatObjectMetadata } from 'src/engine/workspace-manager/workspace-migration/universal-flat-entity/types/universal-flat-object-metadata.type';
 
 const morphIdByRelationObjectNameSingular = {
@@ -50,11 +51,13 @@ export type BuildDefaultRelationFieldsForCustomObjectArgs = {
 type SourceAndTargetFlatFieldMetadatasRecord = {
   standardSourceFlatFieldMetadatas: UniversalFlatFieldMetadata[];
   standardTargetFlatFieldMetadatas: UniversalFlatFieldMetadata[];
+  standardRelationIndexMetadatas: UniversalFlatIndexMetadata[];
 };
 const EMPTY_SOURCE_AND_TARGET_FLAT_FIELD_METADATAS_RECORD: SourceAndTargetFlatFieldMetadatasRecord =
   {
     standardSourceFlatFieldMetadatas: [],
     standardTargetFlatFieldMetadatas: [],
+    standardRelationIndexMetadatas: [],
   };
 
 export const buildDefaultRelationFlatFieldMetadatasForCustomObject = ({
@@ -127,7 +130,7 @@ export const buildDefaultRelationFlatFieldMetadatasForCustomObject = ({
         const morphId =
           morphIdByRelationObjectNameSingular[objectMetadataNameSingular];
 
-        const { flatFieldMetadatas } =
+        const { flatFieldMetadatas, indexMetadatas } =
           generateMorphOrRelationFlatFieldMetadataPair({
             sourceFlatObjectMetadata,
             targetFlatObjectMetadata,
@@ -164,6 +167,10 @@ export const buildDefaultRelationFlatFieldMetadatasForCustomObject = ({
             ...sourceAndTargetFlatFieldMetadatasRecord.standardTargetFlatFieldMetadatas,
             flatFieldMetadatas[1],
           ],
+          standardRelationIndexMetadatas: [
+            ...sourceAndTargetFlatFieldMetadatasRecord.standardRelationIndexMetadatas,
+            ...indexMetadatas,
+          ],
         };
       },
       EMPTY_SOURCE_AND_TARGET_FLAT_FIELD_METADATAS_RECORD,