🐛 Bug: Work items created via the external API with inline assignees never subscribe or notify the assignees

Is there an existing issue for this?

• I have searched the existing issues

Current behavior

Creating a work item through the external REST API (POST /api/v1/workspaces/:slug/projects/:project_id/work-items/) with assignees included in the create payload ("assignees": [...]) silently drops the assignee tracking: no assignee activity is recorded, the assignees are not auto-subscribed to the work item, and consequently they get no notification for the assignment — nor for anything that happens on the work item later, since they never became subscribers.

Assigning the same user in a separate PATCH call afterwards works correctly (activity + auto-subscribe + notification).

Root cause: create_issue_activity in apps/api/plane/bgtasks/issue_activities_task.py only calls track_assignees when the payload contains assignee_ids (the web app field name):

if requested_data.get("assignee_ids") is not None:

The external API serializer uses assignees instead. track_assignees itself already reads both keys via extract_ids(requested_data, "assignee_ids", "assignees"), and the update path maps both field names to track_assignees — only the create gate misses the external API key.

Note: with #9307 the create call now dispatches notification=True, but that alone doesn't help this case — the notification fan-out iterates the created activities and the subscribers, and inline assignees on create produce neither.

Steps to reproduce

1. POST /api/v1/workspaces/:slug/projects/:project_id/work-items/ with {"name": "Test", "assignees": ["<user-uuid>"]} using an API token.
2. Check the work item's activity: there is no "added assignee" entry.
3. Check issue_subscribers: the assignee was not subscribed.
4. The assignee receives no in-app notification and no email, and won't receive any for future changes either.

Environment

Self-hosted Community Edition, reproduced on v1.3.1; the gate is unchanged on current preview.

Expected behavior

Creating a work item with inline assignees via the external API behaves like the web app: assignee activity is tracked, assignees are auto-subscribed, and they are notified.

Start with apps/api/plane/bgtasks/issue_activities_task.py and trace create_issue_activity from POST /api/v1/workspaces/:slug/projects/:project_id/work-items/. Compare the create gate with the existing update path and track_assignees handling for assignees. Done means inline external-API assignees produce activity, subscriptions, and notifications like web-app assignments.