perf(server): index messageChannel/calendarChannel for per-workspace sync crons (#20678)

## Summary

The messaging/calendar import crons each iterate every active workspace
and execute one `find` per workspace against `core."messageChannel"` /
`core."calendarChannel"` with the shape:

```
WHERE "workspaceId" = $1 AND "isSyncEnabled" = true AND "syncStage" = $2 [AND "type" <> $3]
```

There is currently no index supporting that shape, so the planner does a
seq scan on each table for every iteration. On prod-eu (RDS Performance
Insights, `rds-prod-eu-one`), these two queries are the top two by load
— together ~12 AAS, ~12 calls/sec — and have been the primary
contributor to the sustained 100% CPU since active workspace count grew.

This PR adds composite indexes on `(workspaceId, isSyncEnabled,
syncStage)` for both tables as an instance migration in 2.6.0.

packages/twenty-server/src/engine/metadata-modules/calendar-channel/entities/calendar-channel.entity.ts

@@ -4,6 +4,7 @@ import {
   Column,
   CreateDateColumn,
   Entity,
+  Index,
   JoinColumn,
   ManyToOne,
   PrimaryGeneratedColumn,
@@ -35,6 +36,11 @@ registerEnumType(CalendarChannelContactAutoCreationPolicy, {
 });
 
 @Entity({ name: 'calendarChannel', schema: 'core' })
+@Index('IDX_CALENDAR_CHANNEL_WORKSPACE_ID_SYNC_ENABLED_SYNC_STAGE', [
+  'workspaceId',
+  'isSyncEnabled',
+  'syncStage',
+])
 export class CalendarChannelEntity extends WorkspaceRelatedEntity {
   @PrimaryGeneratedColumn('uuid')
   id: string;

packages/twenty-server/src/engine/metadata-modules/message-channel/entities/message-channel.entity.ts

@@ -4,6 +4,7 @@ import {
   Column,
   CreateDateColumn,
   Entity,
+  Index,
   JoinColumn,
   ManyToOne,
   OneToMany,
@@ -47,6 +48,11 @@ registerEnumType(MessageChannelPendingGroupEmailsAction, {
 });
 
 @Entity({ name: 'messageChannel', schema: 'core' })
+@Index('IDX_MESSAGE_CHANNEL_WORKSPACE_ID_SYNC_ENABLED_SYNC_STAGE', [
+  'workspaceId',
+  'isSyncEnabled',
+  'syncStage',
+])
 export class MessageChannelEntity extends WorkspaceRelatedEntity {
   @PrimaryGeneratedColumn('uuid')
   id: string;