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/application/src/lib/issues.service.ts

@@ -2,6 +2,7 @@ import { Injectable } from '@nestjs/common';
 import { CommandBus, QueryBus } from '@nestjs/cqrs';
 import {
   ActivityReadModel,
+  EpicReadModel,
   IssueDetails,
   IssueLinkType,
   IssueReadModel,
@@ -13,6 +14,12 @@ import {
 } from './commands/attach-file.command';
 import { DetachFileCommand } from './commands/detach-file.command';
 import { WatchIssueCommand } from './commands/watch-issue.command';
+import { SetIssueParentCommand } from './commands/set-issue-parent.command';
+import {
+  AddChecklistItemCommand,
+  RemoveChecklistItemCommand,
+  UpdateChecklistItemCommand,
+} from './commands/change-checklist.command';
 import { ChangeIssueStatusCommand } from './commands/change-issue-status.command';
 import { CommandResult } from './commands/command-result';
 import { CreateIssueCommand } from './commands/create-issue.command';
@@ -29,6 +36,7 @@ import {
 import { GetIssueQuery } from './queries/get-issue.query';
 import { GetIssuesByIdsQuery } from './queries/get-issues-by-ids.query';
 import { SearchIssuesQuery } from './queries/search-issues.query';
+import { GetEpicsQuery } from './queries/get-epics.query';
 import {
   MyIssuesFilter,
   SearchMyIssuesQuery,
@@ -41,6 +49,8 @@ export interface CreateIssueInput extends Omit<IssueDetails, 'title' | 'type'> {
   title: string;
   assigneeId?: string | null;
   releaseId?: string | null;
+  /** Epic or ticket the new one breaks down. */
+  parentId?: string | null;
 }
 
 export interface IssueLinkInput {
@@ -57,7 +67,15 @@ export class IssuesService {
   ) {}
 
   create(actorId: string, input: CreateIssueInput): Promise<CommandResult> {
-    const { projectId, type, title, assigneeId, releaseId, ...details } = input;
+    const {
+      projectId,
+      type,
+      title,
+      assigneeId,
+      releaseId,
+      parentId,
+      ...details
+    } = input;
     return this.commandBus.execute(
       new CreateIssueCommand(
         actorId,
@@ -67,6 +85,7 @@ export class IssuesService {
         details,
         assigneeId,
         releaseId,
+        parentId,
       ),
     );
   }
@@ -188,6 +207,52 @@ export class IssuesService {
     return this.queryBus.execute(new GetIssuesByIdsQuery(viewerId, ids));
   }
 
+  /** Places the ticket under `parentId` (null: back to the top). */
+  setParent(
+    actorId: string,
+    issueId: string,
+    parentId: string | null,
+  ): Promise<CommandResult> {
+    return this.commandBus.execute(
+      new SetIssueParentCommand(actorId, issueId, parentId),
+    );
+  }
+
+  addChecklistItem(
+    actorId: string,
+    issueId: string,
+    text: string,
+  ): Promise<CommandResult> {
+    return this.commandBus.execute(
+      new AddChecklistItemCommand(actorId, issueId, text),
+    );
+  }
+
+  updateChecklistItem(
+    actorId: string,
+    issueId: string,
+    itemId: string,
+    changes: { text?: string; done?: boolean },
+  ): Promise<CommandResult> {
+    return this.commandBus.execute(
+      new UpdateChecklistItemCommand(actorId, issueId, itemId, changes),
+    );
+  }
+
+  removeChecklistItem(
+    actorId: string,
+    issueId: string,
+    itemId: string,
+  ): Promise<CommandResult> {
+    return this.commandBus.execute(
+      new RemoveChecklistItemCommand(actorId, issueId, itemId),
+    );
+  }
+
+  epics(viewerId: string | null, projectId: string): Promise<EpicReadModel[]> {
+    return this.queryBus.execute(new GetEpicsQuery(viewerId, projectId));
+  }
+
   search(
     viewerId: string | null,
     filter: IssueFilter,