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/presenters/src/lib/http/issues.controller.ts

@@ -33,6 +33,7 @@ import {
   CommentsService,
   IssuesService,
   PinsService,
+  TreeFollowsService,
   PresenceService,
   ProjectsService,
 } from '@task-stream/api/tracker/application';
@@ -99,6 +100,7 @@ export class IssuesController {
     private readonly presence: PresenceService,
     private readonly projects: ProjectsService,
     private readonly pins: PinsService,
+    private readonly treeFollows: TreeFollowsService,
   ) {}
 
   @Get('me/home')
@@ -203,6 +205,38 @@ export class IssuesController {
     await this.pins.pin(me.id, await this.refs.issueId(me.id, issue), false);
   }
 
+  @Put('issues/:issue/follow-tree')
+  @HttpCode(204)
+  @ISSUE
+  @ApiOperation({
+    summary: 'Follow a ticket with everything under it (an epic)',
+    description:
+      'Told of its changes and of every ticket hanging from it, at any depth, also the ones added later. Private to me.',
+  })
+  @ApiNoContentResponse()
+  @ApiCommandErrors()
+  async followTree(@CurrentUser() me: User, @Param('issue') issue: string) {
+    await this.treeFollows.follow(
+      me.id,
+      await this.refs.issueId(me.id, issue),
+      true,
+    );
+  }
+
+  @Delete('issues/:issue/follow-tree')
+  @HttpCode(204)
+  @ISSUE
+  @ApiOperation({ summary: 'Stop following a ticket with everything under it' })
+  @ApiNoContentResponse()
+  @ApiCommandErrors()
+  async unfollowTree(@CurrentUser() me: User, @Param('issue') issue: string) {
+    await this.treeFollows.follow(
+      me.id,
+      await this.refs.issueId(me.id, issue),
+      false,
+    );
+  }
+
   @Put('issues/:issue/watch')
   @ISSUE
   @ApiOperation({