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/domain/src/lib/ports/gateways.ts

@@ -4,6 +4,7 @@ import {
   Attachment,
   BoardColumn,
   Comment,
+  Epic,
   CommandResult,
   Issue,
   IssuePage,
@@ -89,6 +90,30 @@ export abstract class IssuesGateway {
   ): Observable<CommandResult>;
   /** The signed-in person follows (or stops following) the ticket. */
   abstract watch(issueId: string, watching: boolean): Observable<CommandResult>;
+  /** Tickets hanging from `issueId`. */
+  abstract children(
+    projectId: string,
+    issueId: string,
+  ): Observable<IssueSummary[]>;
+  /** Places the ticket under `parentId` (null: back to the top). */
+  abstract setParent(
+    issueId: string,
+    parentId: string | null,
+  ): Observable<CommandResult>;
+  abstract epics(projectId: string): Observable<Epic[]>;
+  abstract addChecklistItem(
+    issueId: string,
+    text: string,
+  ): Observable<CommandResult>;
+  abstract updateChecklistItem(
+    issueId: string,
+    itemId: string,
+    changes: { text?: string; done?: boolean },
+  ): Observable<CommandResult>;
+  abstract removeChecklistItem(
+    issueId: string,
+    itemId: string,
+  ): Observable<CommandResult>;
 }
 
 export abstract class CommentsGateway {