feat: presence on tickets

A ticket's page beats every 10 s (touchPresence / PUT
/issues/{issue}/presence, with whether I am editing) and gets back who
else is there, editing ones first. Entries live in Redis, one hash per
ticket, and fade after 30 s without a beat; leaving the page takes me out
at once. Only for people who can read the ticket; never in the event
store. The ticket shows «Bob está editando · Ana también lo está viendo»
above its title; editing means a ticket or comment form is open or a
comment is being written.

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

apps/api/e2e/tracker.e2e-spec.ts

@@ -1700,4 +1700,59 @@ describe('task-stream (e2e)', () => {
         .summary,
     ).toMatch(/tu equipo Frontend/);
   });
+
+  it('tells who else is on a ticket, and who is editing it', async () => {
+    const key = 'P' + Date.now().toString(36).slice(-5).toUpperCase();
+    await rest('token-ana', 'POST', '/projects', { key, name: key });
+    await eventually(
+      () => rest('token-ana', 'GET', `/projects/${key}`),
+      (r) => r.status === 200,
+    );
+    await rest('token-ana', 'POST', `/projects/${key}/members`, {
+      userId: '102',
+    });
+    const created = await rest('token-ana', 'POST', `/projects/${key}/issues`, {
+      type: 'TASK',
+      title: 'Aquí estamos',
+    });
+    const issue = await eventually(
+      () => rest('token-bob', 'GET', `/issues/${created.body['key']}`),
+      (r) => r.status === 200,
+    );
+    const id = issue.body['id'];
+
+    expect(
+      (
+        await rest('token-ana', 'PUT', `/issues/${id}/presence`, {
+          editing: false,
+        })
+      ).body,
+    ).toEqual([]);
+    const { touchPresence } = await gql(
+      'token-bob',
+      `mutation($i: ID!) { touchPresence(issueId: $i, editing: true) { user { id name } editing } }`,
+      { i: id },
+    );
+    expect(touchPresence).toEqual([
+      { user: { id: '101', name: 'Ana (lead)' }, editing: false },
+    ]);
+    const seen = await rest('token-ana', 'PUT', `/issues/${id}/presence`, {});
+    expect(seen.body).toEqual([
+      expect.objectContaining({
+        user: { id: '102', name: 'Bob (frontend)' },
+        editing: true,
+      }),
+    ]);
+    // Outsiders cannot even ask (a private ticket does not exist for them).
+    expect(
+      (await rest('token-eve', 'PUT', `/issues/${id}/presence`, {})).status,
+    ).toBe(404);
+
+    expect(
+      (await rest('token-bob', 'DELETE', `/issues/${id}/presence`)).status,
+    ).toBe(204);
+    expect(
+      (await rest('token-ana', 'PUT', `/issues/${id}/presence`, {})).body,
+    ).toEqual([]);
+  });
 });