feat: follow an epic with everything under it

«Seguir todo» on a ticket with subtasks (and on each epic of the Epics
page) tells you of it and of every ticket hanging from it, at any depth,
also the ones added later: their moves, assignments, comments and plan
changes, as a follower of each, plus when a ticket is placed under it.
Guests of a private project are left out. Personal, no history (Mongo
`tree_follows`).

API: followTree / unfollowTree, Issue.followingTree, PUT/DELETE
/api/issues/{issue}/follow-tree. MCP: watch_issue with tree.

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

libs/api/tracker/infrastructure/src/lib/mongo/read-model.schemas.ts

@@ -109,6 +109,7 @@ export const ReadModels = {
   PinnedIssue: 'PinnedIssue',
   AlertIntegration: 'AlertIntegration',
   AlertTrace: 'AlertTrace',
+  TreeFollow: 'TreeFollow',
   Sprint: 'SprintReadModel',
   GitIntegration: 'GitIntegration',
   OutgoingWebhook: 'OutgoingWebhook',
@@ -360,6 +361,15 @@ export const AlertTraceSchema =
   SchemaFactory.createForClass(AlertTraceDocument);
 AlertTraceSchema.index({ projectId: 1, fingerprint: 1 }, { unique: true });
 
+@Schema({ ...LOOSE, collection: 'tree_follows' })
+class TreeFollowDocument {
+  @Prop({ type: String }) userId!: string;
+  @Prop({ type: String, index: true }) issueId!: string;
+}
+export const TreeFollowSchema =
+  SchemaFactory.createForClass(TreeFollowDocument);
+TreeFollowSchema.index({ userId: 1, issueId: 1 }, { unique: true });
+
 @Schema({ ...LOOSE, collection: 'git_integrations' })
 class GitIntegrationDocument {
   /** The project's id. */
@@ -465,6 +475,7 @@ export const READ_MODEL_SCHEMAS = [
   { name: ReadModels.PinnedIssue, schema: PinnedIssueSchema },
   { name: ReadModels.AlertIntegration, schema: AlertIntegrationSchema },
   { name: ReadModels.AlertTrace, schema: AlertTraceSchema },
+  { name: ReadModels.TreeFollow, schema: TreeFollowSchema },
   { name: ReadModels.CalendarFeed, schema: CalendarFeedSchema },
   { name: ReadModels.DashboardShare, schema: DashboardShareSchema },
   { name: ReadModels.PushSubscription, schema: PushSubscriptionSchema },

libs/api/tracker/infrastructure/src/lib/mongo/tree-follows.repository.ts

@@ -0,0 +1,39 @@
+import { Injectable } from '@nestjs/common';
+import { InjectModel } from '@nestjs/mongoose';
+import {
+  TreeFollow,
+  TreeFollowsRepository,
+} from '@task-stream/api/tracker/application';
+import { Model } from 'mongoose';
+import { ReadModels } from './read-model.schemas';
+
+@Injectable()
+export class MongoTreeFollowsRepository extends TreeFollowsRepository {
+  constructor(
+    @InjectModel(ReadModels.TreeFollow)
+    private readonly follows: Model<TreeFollow>,
+  ) {
+    super();
+  }
+
+  async follow(follow: TreeFollow): Promise<void> {
+    await this.follows.updateOne(
+      { userId: follow.userId, issueId: follow.issueId },
+      { $setOnInsert: follow },
+      { upsert: true },
+    );
+  }
+
+  async unfollow(userId: string, issueId: string): Promise<void> {
+    await this.follows.deleteOne({ userId, issueId });
+  }
+
+  async isFollowing(userId: string, issueId: string): Promise<boolean> {
+    return !!(await this.follows.exists({ userId, issueId }));
+  }
+
+  async followersOf(issueIds: string[]): Promise<string[]> {
+    if (!issueIds.length) return [];
+    return this.follows.distinct('userId', { issueId: { $in: issueIds } });
+  }
+}