nested subtasks on the kanban board with cascade move

Motivation

Right now, if a task has subtasks (via taskRelationTable with relationType = \"subtask\"), the parent/child relationship is only visible on the task detail page (apps/web/src/components/task/task-subtasks.tsx). On the kanban board the subtasks render as independent, top-level cards, so:

1. The board gives no visual hint that a card is a subtask of another.
2. Moving a parent between columns does not move its subtasks along with it, which is the expected Jira-style behaviour most users reach for.

Current state of the code

Good news — a lot of the plumbing is already there:

• Backend relations — taskRelationTable exists in apps/api/src/database/schema.ts and the \"subtask\" relation type is already accepted by the validator in apps/api/src/task-relation/index.ts. CRUD controllers for relations are implemented.
• Subtask panel on task detail — apps/web/src/components/task/task-subtasks.tsx already loads and filters relationType === \"subtask\" && sourceTaskId === taskId and has a UI for adding/removing subtasks.

What's missing:

• apps/api/src/task/controllers/get-tasks.ts does not join taskRelationTable, so the board endpoint has no idea which tasks are subtasks of other tasks in the same project.
• apps/web/src/components/kanban-board/column/column-dropzone.tsx renders every task in the column without filtering subtasks out.
• apps/web/src/components/kanban-board/task-card.tsx has no concept of child cards.
• apps/web/src/components/kanban-board/index.tsx handleDragEnd only updates the dragged task — no cascade to the subtasks.
• apps/api/src/task/controllers/update-task-status.ts and the updateStatus branch of apps/api/src/task/controllers/bulk-update-tasks.ts also don't cascade.

Proposed approach

Option A — hide + cascade (MVP, smaller change)

1. Extend get-tasks.ts to include parent-relation info per task (e.g. a parentTaskId field, derived from a join on taskRelationTable).
2. In column-dropzone.tsx, skip rendering tasks whose parentTaskId is present and points to a task in the same project. They're still reachable from the parent's detail page.
3. In update-task-status.ts (and the bulk-update-tasks status branch), cascade the status change to all subtasks whose parent is the task being updated.
4. In kanban-board/index.tsx handleDragEnd, after updating the parent, fire the same update for each subtask so the optimistic cache stays in sync.

This is the recommendation for a first iteration — it's invasive but contained, and the UX is "subtasks live on the task page, move with the parent implicitly".

Option B — nested cards (what the requester actually asked for)

On top of option A:
5. In task-card.tsx, render a compact list of subtasks (titles + status dots + a progress ring) directly under the parent title — similar to how task-subtasks.tsx already renders them on the detail page.
6. Figure out drag behaviour for the inner list. Two reasonable rules:

• A subtask is draggable only inside its parent card; dropping it outside re-parents it (or promotes it to a top-level task).
• Or: subtasks are read-only on the board and can only be rearranged from the task detail page.

7. Decide how to style nesting (indentation, hairline connector, collapsed/expanded state, localStorage for collapse per parent).

Option B is roughly 1–2 weeks of frontend work. Option A is 3–5 days.

UX questions worth answering before coding

• Should subtasks keep an independent status, or should they always match the parent's column?
• If a subtask is assigned to someone different from the parent, who gets the notification on status cascade — the subtask assignee, both, or neither?
• If the parent is moved to a different project via move-task, should its subtasks follow? (Currently they wouldn't — move-task.ts only updates the dragged task.)
• Collapsed state of parent cards: per-user setting, per-device (localStorage), or always expanded?

Acceptance criteria

• Subtasks no longer appear as independent cards on the kanban board when their parent lives in the same project.
• Dragging a parent card to a new column moves all its subtasks to the same column, both optimistically on the frontend and durably on the backend.
• Bulk status updates on a parent cascade to subtasks.
• (Option B) Subtasks render visually nested under their parent card on the board.
• No regression on the existing subtasks panel in task-subtasks.tsx.

Related

• Fix: #1165 (the task move popover crash this report was written against)
• Feature: #1166 (task author sticker — independent but in the same sprint)

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Originally requested by @RainyPixel on 2026-04-07. Original GitHub request #1170.