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/infrastructure/src/lib/graphql/issues.graphql

@@ -129,3 +129,50 @@ query MyIssues(
     }
   }
 }
+
+query IssueChildren($filter: IssueFilterInput!) {
+  issues(filter: $filter, limit: 200) {
+    items {
+      ...IssueSummaryFields
+    }
+  }
+}
+
+query Epics($projectId: ID!) {
+  epics(projectId: $projectId) {
+    issue {
+      ...IssueSummaryFields
+    }
+    progress {
+      total
+      finished
+      percent
+      storyPoints
+      finishedStoryPoints
+    }
+  }
+}
+
+mutation SetIssueParent($issueId: ID!, $parentId: ID) {
+  setIssueParent(issueId: $issueId, parentId: $parentId) {
+    ...CommandResultFields
+  }
+}
+
+mutation AddChecklistItem($input: AddChecklistItemInput!) {
+  addChecklistItem(input: $input) {
+    ...CommandResultFields
+  }
+}
+
+mutation UpdateChecklistItem($input: UpdateChecklistItemInput!) {
+  updateChecklistItem(input: $input) {
+    ...CommandResultFields
+  }
+}
+
+mutation RemoveChecklistItem($issueId: ID!, $itemId: ID!) {
+  removeChecklistItem(issueId: $issueId, itemId: $itemId) {
+    ...CommandResultFields
+  }
+}