feat: epics, subtasks and checklists

Tickets can hang from another one of the same project, three levels deep
at most (epic → ticket → subtask). The ticket page lists its subtasks with
their progress, creates new ones and moves the ticket under an epic; a new
Épicas tab shows every epic and how far it has got. Each ticket also gets a
checklist for the small steps that don't deserve a ticket.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/domain/src/lib/issue/issue.ts

@@ -7,9 +7,13 @@ import {
 import { IssueAssignedEvent } from '../events/issue-assigned.event';
 import { IssueAttachmentAddedEvent } from '../events/issue-attachment-added.event';
 import { IssueAttachmentRemovedEvent } from '../events/issue-attachment-removed.event';
+import { IssueChecklistItemAddedEvent } from '../events/issue-checklist-item-added.event';
+import { IssueChecklistItemRemovedEvent } from '../events/issue-checklist-item-removed.event';
+import { IssueChecklistItemUpdatedEvent } from '../events/issue-checklist-item-updated.event';
 import { IssueCreatedEvent } from '../events/issue-created.event';
 import { IssueDeletedEvent } from '../events/issue-deleted.event';
 import { IssueLinkedEvent } from '../events/issue-linked.event';
+import { IssueParentChangedEvent } from '../events/issue-parent-changed.event';
 import { IssueReleaseChangedEvent } from '../events/issue-release-changed.event';
 import { IssueStatusChangedEvent } from '../events/issue-status-changed.event';
 import { IssueUnlinkedEvent } from '../events/issue-unlinked.event';
@@ -22,6 +26,12 @@ import {
   MAX_ATTACHMENTS_PER_ISSUE,
   normalizeAttachment,
 } from './issue-attachment';
+import {
+  ChecklistItem,
+  MAX_CHECKLIST_ITEMS,
+  normalizeChecklistText,
+} from './issue-checklist';
+import { ParentCandidate, assertCanNest } from './issue-hierarchy';
 import { IssueLinkType, IssuePriority } from './issue.enums';
 
 export interface IssueLink {
@@ -95,6 +105,9 @@ export class Issue extends VersionedAggregateRoot implements IssueProps {
   public attachments: IssueAttachment[] = [];
   /** People who asked to be told about this ticket. */
   public watchers: string[] = [];
+  /** The epic or ticket this one breaks down. */
+  public parentId: string | null = null;
+  public checklist: ChecklistItem[] = [];
   public createdAt!: string;
   public updatedAt!: string;
   public deleted = false;
@@ -280,6 +293,91 @@ export class Issue extends VersionedAggregateRoot implements IssueProps {
     );
   }
 
+  /**
+   * Places the ticket under `parent` (null takes it out). `childHeight` is
+   * how many levels of tickets already hang from this one.
+   */
+  setParent(
+    actorId: string,
+    parent: ParentCandidate | null,
+    childHeight = 0,
+  ): void {
+    if ((parent?.id ?? null) === this.parentId) return;
+    if (parent) assertCanNest(this, parent, childHeight);
+    this.apply(
+      new IssueParentChangedEvent(
+        actorId,
+        this.id,
+        this.projectId,
+        parent?.id ?? null,
+        parent?.key ?? null,
+        this.parentId,
+      ),
+    );
+  }
+
+  /** Adds a step to the checklist; `id` is generated by the caller. */
+  addChecklistItem(actorId: string, id: string, text: string): void {
+    if (this.checklist.some((i) => i.id === id)) return;
+    if (this.checklist.length >= MAX_CHECKLIST_ITEMS) {
+      throw new BusinessRuleError(
+        `${this.key} already has ${MAX_CHECKLIST_ITEMS} checklist items`,
+      );
+    }
+    this.apply(
+      new IssueChecklistItemAddedEvent(actorId, this.id, this.projectId, {
+        id,
+        text: normalizeChecklistText(text),
+        done: false,
+      }),
+    );
+  }
+
+  /** Rewords, ticks or unticks a step. Nothing changes, nothing happens. */
+  updateChecklistItem(
+    actorId: string,
+    itemId: string,
+    changes: { text?: string; done?: boolean },
+  ): void {
+    const item = this.checklistItem(itemId);
+    const patch: { text?: string; done?: boolean } = {};
+    if (changes.text !== undefined) {
+      const text = normalizeChecklistText(changes.text);
+      if (text !== item.text) patch.text = text;
+    }
+    if (changes.done !== undefined && changes.done !== item.done)
+      patch.done = changes.done;
+    if (Object.keys(patch).length === 0) return;
+    this.apply(
+      new IssueChecklistItemUpdatedEvent(
+        actorId,
+        this.id,
+        this.projectId,
+        itemId,
+        patch,
+      ),
+    );
+  }
+
+  removeChecklistItem(actorId: string, itemId: string): void {
+    const item = this.checklistItem(itemId);
+    this.apply(
+      new IssueChecklistItemRemovedEvent(
+        actorId,
+        this.id,
+        this.projectId,
+        item.id,
+        item.text,
+      ),
+    );
+  }
+
+  private checklistItem(itemId: string): ChecklistItem {
+    const item = this.checklist.find((i) => i.id === itemId);
+    if (!item) throw new BusinessRuleError('Checklist item not found');
+    return item;
+  }
+
   /** Deletes the ticket (its project was deleted). Idempotent. */
   delete(actorId: string): void {
     if (this.deleted) return;
@@ -360,6 +458,32 @@ export class Issue extends VersionedAggregateRoot implements IssueProps {
     this.watchers = this.watchers.filter((id) => id !== event.userId);
   }
 
+  onIssueParentChangedEvent(event: IssueParentChangedEvent): void {
+    this.parentId = event.parentId;
+    this.updatedAt = event.occurredAt;
+  }
+
+  onIssueChecklistItemAddedEvent(event: IssueChecklistItemAddedEvent): void {
+    this.checklist = [...this.checklist, { ...event.item }];
+    this.updatedAt = event.occurredAt;
+  }
+
+  onIssueChecklistItemUpdatedEvent(
+    event: IssueChecklistItemUpdatedEvent,
+  ): void {
+    this.checklist = this.checklist.map((i) =>
+      i.id === event.itemId ? { ...i, ...event.changes } : i,
+    );
+    this.updatedAt = event.occurredAt;
+  }
+
+  onIssueChecklistItemRemovedEvent(
+    event: IssueChecklistItemRemovedEvent,
+  ): void {
+    this.checklist = this.checklist.filter((i) => i.id !== event.itemId);
+    this.updatedAt = event.occurredAt;
+  }
+
   onIssueDeletedEvent(): void {
     this.deleted = true;
   }