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/fragments.graphql

@@ -69,6 +69,11 @@ fragment IssueSummaryFields on Issue {
     id
     name
   }
+  parent {
+    id
+    key
+    title
+  }
 }
 
 fragment IssueFields on Issue {
@@ -91,6 +96,11 @@ fragment IssueFields on Issue {
   watchers {
     ...UserRefFields
   }
+  checklist {
+    id
+    text
+    done
+  }
   attachments {
     id
     fileName