feat: add subtask management features to view settings and API

apps/e2e/features/projects/subtask-hierarchy.feature

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+@projects @interactions @subtasks
+Feature: Hierarchical subtasks in task views
+  Tasks can have children through their parent task.  The list API reports
+  how many direct children a task has (`subtask_count`) and how many of them
+  are in a status of category "done" (`subtask_done_count`).  The Table views
+  (product backlog and sprints) and the Timeline show those parents with a
+  chevron and a "done/total" progress chip; expanding a row loads its
+  children on demand, indented under the parent, using the view's filters and
+  sort.  Board cards show the same chip but do not nest.  One "Subtask
+  setting in the View settings panel of Table and Timeline views chooses
+  "Nested, collapsed" (the default), "Nested, expanded" or "Hidden (flat
+  list)"; it is saved with the view like any other view setting.  Rows the
+  user opens or closes by hand are remembered per user and per view.  System "Subtask"
+  Tasks stay out of the top level of the Table and Timeline while nesting is
+  on and appear only under their parent (the "exclude_subtasks" list filter);
+  tasks that merely belong to an Epic stay in the list.  With nesting off, or
+  on the Board, every task is shown as its own row or card.
+
+  @authenticated
+  Rule: The task list API reports subtask counts and lists children
+
+    Background:
+      Given the user already has a stored authenticated session
+      And a project named "E2E_SUB_API_PROJECT" exists
+      And the project has a task "Parent" with three subtasks, one of them in a "done" status
+      And one more subtask of "Parent" has been deleted
+
+    Scenario: A parent's list entry carries its subtask counts
+      When the task list is requested
+      Then "Parent" should report subtask_count 3 and subtask_done_count 1
+      And a task without subtasks should report no subtask counts
+
+    Scenario: Listing by parent returns only that parent's children
+      When the task list is requested with parent_task_id of "Parent"
+      Then exactly the three live subtasks should be returned
+
+    Scenario: exclude_subtasks hides subtasks but keeps tasks of an Epic
+      Given the project has an Epic with one task
+      When the task list is requested with exclude_subtasks
+      Then the Epic, its task, "Parent" and a task without subtasks should be returned
+      And no subtask of "Parent" should be returned
+
+    Scenario: Children can be filtered by status
+      When the task list is requested with parent_task_id of "Parent" and the "done" status
+      Then only the done subtask should be returned
+
+    Scenario: Listing children ignores the sprint filter
+      Given "Parent" is in a sprint and its subtasks are not
+      When the children of "Parent" are requested together with that sprint filter
+      Then all three subtasks should still be returned
+
+  @authenticated
+  Rule: The product backlog Table nests subtasks under their parents
+
+    Background:
+      Given the user already has a stored authenticated session
+      And a project named "E2E_SUB_TABLE_PROJECT" exists
+      And the project has a task "Parent" with three subtasks, one of them done
+      And the user has navigated to the "Product Backlog" Table view
+
+    Scenario: A parent shows its progress and starts collapsed
+      Then "Parent" should show an "Expand 3 subtasks" chevron and a "1/3" chip
+      And none of its subtasks should be visible
+
+    Scenario: Expanding a parent shows its subtasks and collapsing hides them
+      When the user expands "Parent"
+      Then its three subtasks should be visible
+      When the user collapses "Parent"
+      Then its subtasks should be hidden again
+
+    Scenario: A task without subtasks has no chevron
+      Given the project has a task "Lone" without subtasks
+      Then "Lone" should have no expand chevron and no progress chip
+
+    Scenario: Subtasks do not appear as top-level rows
+      Then no subtask of "Parent" should be listed as a top-level row
+
+    Scenario: A subtask can be added under an expanded parent
+      When the user expands "Parent"
+      And the user adds a subtask "Fourth" with the "Add subtask" button
+      Then "Fourth" should appear under "Parent"
+      And "Parent" should show a "1/4" chip
+
+    Scenario: Clicking the chevron does not open the task
+      When the user expands "Parent"
+      Then the task detail should not be open
+
+    Scenario: Clicking a subtask opens its details
+      When the user expands "Parent"
+      And the user clicks the subtask "Child done"
+      Then the task detail for "Child done" should open
+
+    Scenario: Expanded rows are remembered after a reload
+      When the user expands "Parent"
+      And the page is reloaded
+      Then the subtasks of "Parent" should still be visible
+
+  @authenticated
+  Rule: The Subtasks view setting controls nesting
+
+    Background:
+      Given the user already has a stored authenticated session
+      And a project named "E2E_SUB_SETTING_PROJECT" exists
+      And the project has a task "Parent" with three subtasks, one of them done
+      And the user has navigated to the "Product Backlog" Table view
+
+    Scenario: The setting defaults to "Nested, collapsed"
+      When the user opens the View settings panel
+      Then the "Subtasks" setting should show "Nested, collapsed"
+
+    Scenario: "Nested, expanded" shows subtasks without clicking the chevron
+      When the user chooses "Nested, expanded" and saves only for themselves
+      Then the three subtasks of "Parent" should be visible
+      And "Parent" should show a "Collapse 3 subtasks" chevron
+
+    Scenario: A row can still be collapsed by hand when subtasks start expanded
+      Given the "Subtasks" setting is "Nested, expanded"
+      When the user collapses "Parent"
+      Then its subtasks should be hidden and the chevron should offer to expand
+
+    Scenario: "Hidden (flat list)" removes the chevrons and the chips
+      When the user chooses "Hidden (flat list)" and saves only for themselves
+      Then no parent should show a chevron or a progress chip
+      And the subtasks should be shown as plain top-level rows
+
+    Scenario: Switching back to nested restores the chevrons
+      Given the "Subtasks" setting is "Hidden (flat list)"
+      When the user chooses "Nested, collapsed" and saves only for themselves
+      Then "Parent" should show its chevron and its "1/3" chip
+      And its subtasks should be hidden
+
+    Scenario: The setting is remembered after a reload
+      When the user chooses "Nested, expanded" and saves only for themselves
+      And the page is reloaded
+      Then the subtasks of "Parent" should be visible
+      And the "Subtasks" setting should show "Nested, expanded"
+
+  @authenticated
+  Rule: The Timeline nests tasks under their Epics
+
+    Background:
+      Given the user already has a stored authenticated session
+      And a project named "E2E_SUB_TIMELINE_PROJECT" exists
+      And the project has an Epic "Epic" with two child tasks
+      And the user has navigated to the Timeline interaction
+
+    Scenario: An Epic shows its child count and expands to its tasks
+      Then "Epic" should show an "Expand 2 subtasks" chevron
+      When the user expands "Epic"
+      Then both child tasks should be visible under "Epic"
+
+    Scenario: The Subtasks view setting also applies to the Timeline
+      When the user chooses "Nested, expanded" and saves only for themselves
+      Then both child tasks should be visible under "Epic"
+
+  @authenticated
+  Rule: Board cards show a subtask progress chip
+
+    Background:
+      Given the user already has a stored authenticated session
+      And a project named "E2E_SUB_BOARD_PROJECT" exists
+      And the project has a task "Parent" with three subtasks, one of them done
+      And the user has navigated to the "Product Backlog" Board view
+
+    Scenario: A card with subtasks shows "done/total"
+      Then the "Parent" card should show a "1/3" chip
+
+    Scenario: The Board offers no Subtasks view setting
+      When the user opens the View settings panel
+      Then there should be no "Subtasks" setting
+
+    Scenario: Subtasks stay visible as their own cards
+      Then the subtasks of "Parent" should each be shown as a card