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/web/tracker/application/src/lib/events/issue.events.ts

@@ -6,6 +6,7 @@ import {
   Comment,
   CommentChanged,
   Issue,
+  IssueSummary,
   LinkIssuesInput,
   UpdateIssueInput,
 } from '@task-stream/web/tracker/domain';
@@ -36,6 +37,21 @@ export const issuePageEvents = eventGroup({
       files?: File[];
     }>(),
     watchRequested: type<{ issueId: string; watching: boolean }>(),
+    /** Place the ticket under another one by its key (e.g. WEB-3); null: out. */
+    parentRequested: type<{ issueId: string; parentKey: string | null }>(),
+    subtaskCreateRequested: type<{
+      projectId: string;
+      parentId: string;
+      type: string;
+      title: string;
+    }>(),
+    checklistAddRequested: type<{ issueId: string; text: string }>(),
+    checklistUpdateRequested: type<{
+      issueId: string;
+      itemId: string;
+      changes: { text?: string; done?: boolean };
+    }>(),
+    checklistRemoveRequested: type<{ issueId: string; itemId: string }>(),
     attachRequested: type<{
       issueId: string;
       files: File[];
@@ -57,7 +73,10 @@ export const issueApiEvents = eventGroup({
     loadFailed: type<string>(),
     commentsLoaded: type<Comment[]>(),
     historyLoaded: type<Activity[]>(),
+    childrenLoaded: type<IssueSummary[]>(),
     commandSucceeded: type<string>(),
+    /** A checklist step was saved (no toast for something so small). */
+    checklistSaved: type<void>(),
     commandFailed: type<string>(),
   },
 });
@@ -67,6 +86,8 @@ export const issueRealtimeEvents = eventGroup({
   events: {
     /** Something about this ticket changed: refetch it. */
     issueTouched: type<{ issueId: string; version: number }>(),
+    /** A ticket under this one changed, arrived or left: reload them. */
+    childrenTouched: type<{ issueId: string }>(),
     commentChanged: type<CommentChanged>(),
   },
 });