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/database/commands/upgrade-version-command/instance-commands.constant.ts

@@ -48,6 +48,7 @@ import { EncryptTotpSecretsSlowInstanceCommand } from 'src/database/commands/upg
 import { AddSubFieldNameToViewSortFastInstanceCommand } from 'src/database/commands/upgrade-version-command/2-5/2-5-instance-command-fast-1778502963794-add-sub-field-name-to-view-sort';
 import { DropPostgresCredentialsTableFastInstanceCommand } from 'src/database/commands/upgrade-version-command/2-5/2-5-instance-command-fast-1798500000000-drop-postgres-credentials-table';
 import { AddRelationTargetFieldMetadataIdToViewFilterFastInstanceCommand } from 'src/database/commands/upgrade-version-command/2-6/2-6-instance-command-fast-1798000005000-add-relation-target-field-metadata-id-to-view-filter';
+import { AddChannelSyncStageIndexesFastInstanceCommand } from 'src/database/commands/upgrade-version-command/2-6/2-6-instance-command-fast-1798000010000-add-channel-sync-stage-indexes';
 
 export const INSTANCE_COMMANDS = [
   AddViewFieldGroupIdIndexOnViewFieldFastInstanceCommand,
@@ -98,4 +99,5 @@ export const INSTANCE_COMMANDS = [
   AddSubFieldNameToViewSortFastInstanceCommand,
   DropPostgresCredentialsTableFastInstanceCommand,
   AddRelationTargetFieldMetadataIdToViewFilterFastInstanceCommand,
+  AddChannelSyncStageIndexesFastInstanceCommand,
 ];