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/api/tracker/application/src/lib/ports/find-issues.repository.ts

@@ -1,4 +1,5 @@
 import {
+  EpicReadModel,
   IssuePriority,
   IssueReadModel,
   ProjectStatsReadModel,
@@ -18,6 +19,8 @@ export interface IssueFilter {
   unassigned?: boolean;
   releaseId?: string;
   labels?: string[];
+  /** Only the tickets hanging from this one. */
+  parentId?: string;
   search?: string;
   limit: number;
   offset: number;
@@ -51,6 +54,8 @@ export abstract class FindIssuesRepository {
   abstract findOpenAssignedTo(userId: string): Promise<IssueReadModel[]>;
   /** Every ticket of a project (statistics). */
   abstract findByProject(projectId: string): Promise<IssueReadModel[]>;
+  /** Top-level tickets with others hanging from them, by key. */
+  abstract epics(projectId: string): Promise<EpicReadModel[]>;
   abstract releaseProgress(
     releaseId: string,
   ): Promise<ReleaseProgressReadModel>;

libs/api/tracker/application/src/lib/ports/tracker-streams.repository.ts

@@ -10,4 +10,6 @@ export abstract class TrackerStreamsRepository {
   abstract commentIdsOfIssue(issueId: string): Promise<string[]>;
   /** Tickets that were ever linked to `issueId` (they may have unlinked since). */
   abstract issueIdsLinkingTo(issueId: string): Promise<string[]>;
+  /** Tickets that were ever placed under `issueId` (they may have left since). */
+  abstract issueIdsEverUnder(issueId: string): Promise<string[]>;
 }

libs/api/tracker/application/src/lib/ports/upsert-materialized-issue.repository.ts

@@ -1,5 +1,6 @@
 import {
   AttachmentReadModel,
+  ChecklistItemReadModel,
   IssueLinkReadModel,
   IssueReadModel,
   StatusCategory,
@@ -13,7 +14,7 @@ export abstract class UpsertMaterializedIssueRepository {
       Partial<
         Omit<
           IssueReadModel,
-          'links' | 'commentCount' | 'attachments' | 'watchers'
+          'links' | 'commentCount' | 'attachments' | 'watchers' | 'checklist'
         >
       >,
   ): Promise<void>;
@@ -35,6 +36,19 @@ export abstract class UpsertMaterializedIssueRepository {
   /** Adds a follower (idempotent). */
   abstract addWatcher(issueId: string, user: UserRefReadModel): Promise<void>;
   abstract removeWatcher(issueId: string, userId: string): Promise<void>;
+  /** Adds a checklist step (idempotent). */
+  abstract addChecklistItem(
+    issueId: string,
+    item: ChecklistItemReadModel,
+  ): Promise<void>;
+  abstract updateChecklistItem(
+    issueId: string,
+    itemId: string,
+    changes: Partial<Pick<ChecklistItemReadModel, 'text' | 'done'>>,
+  ): Promise<void>;
+  abstract removeChecklistItem(issueId: string, itemId: string): Promise<void>;
+  /** Keeps the denormalised parent title of a ticket's children in sync. */
+  abstract renameParent(parentId: string, title: string): Promise<void>;
   abstract setCommentCount(
     issueId: string,
     commentCount: number,