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/infrastructure/src/lib/mongo/find.repositories.ts

@@ -14,6 +14,7 @@ import {
 import {
   ActivityReadModel,
   CommentReadModel,
+  EpicReadModel,
   CountBy,
   defaultProjectSettings,
   IssueReadModel,
@@ -228,6 +229,53 @@ export class MongoFindIssuesRepository extends FindIssuesRepository {
     return docs.map(issueFromDoc);
   }
 
+  async epics(projectId: string): Promise<EpicReadModel[]> {
+    const finished = { $in: ['$statusCategory', FINISHED] };
+    const points = { $ifNull: ['$storyPoints', 0] };
+    const rows = await this.issues.aggregate<{
+      _id: string;
+      total: number;
+      finished: number;
+      storyPoints: number;
+      finishedStoryPoints: number;
+    }>([
+      { $match: { projectId, 'parent.id': { $type: 'string' } } },
+      {
+        $group: {
+          _id: '$parent.id',
+          total: { $sum: 1 },
+          finished: { $sum: { $cond: [finished, 1, 0] } },
+          storyPoints: { $sum: points },
+          finishedStoryPoints: { $sum: { $cond: [finished, points, 0] } },
+        },
+      },
+    ]);
+    if (rows.length === 0) return [];
+    const progress = new Map(rows.map((row) => [row._id, row]));
+    // Only the top of each tree: a ticket with subtasks under an epic is not one.
+    const docs = await this.issues
+      .find({ _id: { $in: [...progress.keys()] }, 'parent.id': null })
+      .sort({ number: 1 })
+      .lean<IssueDoc[]>();
+    return docs.map((doc) => {
+      const { _id: _ignored, ...row } = progress.get(doc._id) ?? {
+        _id: doc._id,
+        total: 0,
+        finished: 0,
+        storyPoints: 0,
+        finishedStoryPoints: 0,
+      };
+      return {
+        issue: issueFromDoc(doc),
+        progress: {
+          ...row,
+          percent:
+            row.total === 0 ? 0 : Math.round((row.finished / row.total) * 100),
+        },
+      };
+    });
+  }
+
   async releaseProgress(releaseId: string): Promise<ReleaseProgressReadModel> {
     const rows = await this.issues.aggregate<{
       _id: { status: string; category: StatusCategory };
@@ -268,6 +316,7 @@ function toMongoFilter(filter: IssueFilter): FilterQuery<IssueDoc> {
   if (filter.labels?.length)
     query.labels = { $all: filter.labels.map((l) => l.toLowerCase()) };
   if (filter.releaseId) query['release.id'] = filter.releaseId;
+  if (filter.parentId) query['parent.id'] = filter.parentId;
   if (filter.unassigned) query['assignee'] = null;
   else if (filter.assigneeId) query['assignee.id'] = filter.assigneeId;
   if (filter.search?.trim()) {