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/presenters/src/lib/graphql/issue.resolver.ts

@@ -23,10 +23,12 @@ import {
   StatusCategory,
 } from '@task-stream/api/tracker/domain';
 import {
+  AddChecklistItemInput,
   ChangeIssueStatusInput,
   CreateIssueInput,
   IssueFilterInput,
   LinkIssuesInput,
+  UpdateChecklistItemInput,
   UpdateIssueInput,
 } from './inputs';
 import { withoutNulls } from '../common/issue-input';
@@ -35,6 +37,7 @@ import {
   ActivityGql,
   CommandResultGql,
   CommentPageGql,
+  EpicGql,
   IssueGql,
   IssuePageGql,
 } from './types';
@@ -72,6 +75,18 @@ export class IssueResolver {
     });
   }
 
+  @OptionalAuth()
+  @Query(() => [EpicGql], {
+    description:
+      "A project's epics: top-level tickets that group others, with the progress of those",
+  })
+  epics(
+    @CurrentUser() me: User | undefined,
+    @Args('projectId', { type: () => ID }) projectId: string,
+  ) {
+    return this.issuesService.epics(viewerOf(me), projectId);
+  }
+
   @Query(() => IssuePageGql, {
     description:
       'Tickets across all my projects: assigned to me (default) or any (e.g. categories [QA])',
@@ -238,6 +253,56 @@ export class IssueResolver {
     return this.issuesService.unlink(me.id, input);
   }
 
+  @Mutation(() => CommandResultGql, {
+    description:
+      'Place a ticket under an epic or another ticket of the same project (null: back to the top). Three levels at most: epic → ticket → subtask.',
+  })
+  setIssueParent(
+    @CurrentUser() me: User,
+    @Args('issueId', { type: () => ID }) issueId: string,
+    @Args('parentId', { type: () => ID, nullable: true })
+    parentId: string | null,
+  ) {
+    return this.issuesService.setParent(me.id, issueId, parentId ?? null);
+  }
+
+  @Mutation(() => CommandResultGql, {
+    description: "Add a step to a ticket's checklist",
+  })
+  addChecklistItem(
+    @CurrentUser() me: User,
+    @Args('input') input: AddChecklistItemInput,
+  ) {
+    return this.issuesService.addChecklistItem(
+      me.id,
+      input.issueId,
+      input.text,
+    );
+  }
+
+  @Mutation(() => CommandResultGql, {
+    description: 'Reword, tick or untick a checklist step',
+  })
+  updateChecklistItem(
+    @CurrentUser() me: User,
+    @Args('input') input: UpdateChecklistItemInput,
+  ) {
+    const { issueId, itemId, text, done } = input;
+    return this.issuesService.updateChecklistItem(me.id, issueId, itemId, {
+      ...(text != null && { text }),
+      ...(done != null && { done }),
+    });
+  }
+
+  @Mutation(() => CommandResultGql)
+  removeChecklistItem(
+    @CurrentUser() me: User,
+    @Args('issueId', { type: () => ID }) issueId: string,
+    @Args('itemId', { type: () => ID }) itemId: string,
+  ) {
+    return this.issuesService.removeChecklistItem(me.id, issueId, itemId);
+  }
+
   @Mutation(() => CommandResultGql, {
     description: 'Put a ticket in a release (null removes it)',
   })