feat(issues): add sub-issue support (#483)

* feat(issues): add sub-issue support

- Backend: Add ListChildIssues SQL query, add parent_issue_id to UpdateIssue,
  add GET /api/issues/{id}/children endpoint
- Frontend: Display parent issue breadcrumb and link in issue detail sidebar,
  show child issues list with status icons, add "Create sub-issue" action in
  dropdown menu and sidebar, pass parent_issue_id through create issue modal
- Update test mocks for new API method

* fix(issues): add parent validation, cycle detection, and improve child refresh

- CreateIssue: validate parent issue exists in the same workspace
- UpdateIssue: validate parent exists, prevent self-referencing, detect
  circular parent chains (up to 10 levels deep)
- Frontend: derive child issues from store when available instead of
  refetching on every global issue count change

apps/web/features/modals/create-issue.tsx

@@ -142,6 +142,7 @@ export function CreateIssueModal({ onClose, data }: { onClose: () => void; data?
         assignee_id: assigneeId,
         due_date: dueDate || undefined,
         attachment_ids: attachmentIds.length > 0 ? attachmentIds : undefined,
+        parent_issue_id: (data?.parent_issue_id as string) || undefined,
       });
       clearDraft();
       onClose();
@@ -196,7 +197,13 @@ export function CreateIssueModal({ onClose, data }: { onClose: () => void; data?
           <div className="flex items-center gap-1.5 text-xs">
             <span className="text-muted-foreground">{workspaceName}</span>
             <ChevronRight className="size-3 text-muted-foreground/50" />
-            <span className="font-medium">New issue</span>
+            {typeof data?.parent_issue_identifier === "string" && (
+              <>
+                <span className="text-muted-foreground">{data.parent_issue_identifier}</span>
+                <ChevronRight className="size-3 text-muted-foreground/50" />
+              </>
+            )}
+            <span className="font-medium">{data?.parent_issue_id ? "New sub-issue" : "New issue"}</span>
           </div>
           <div className="flex items-center gap-1">
             <Tooltip>