feat(backend): activate endpoint for deactivated users

Symmetrisch zu Modul-27/28-Fix fuer Colony/Apiary: kein Weg, einen
versehentlich deaktivierten Benutzer wieder zu aktivieren. Neuer
POST :id/activate-Endpunkt setzt isActive zurueck auf true und
loescht deactivatedAt; ConflictException wenn bereits aktiv.

ApiCore-Backend/test/users.e2e-spec.ts

@@ -99,4 +99,19 @@ describe('Users (e2e)', () => {
     expect(deactivated.isActive).toBe(false);
     expect(deactivated.deactivatedAt).not.toBeNull();
   });
+
+  it('lets an admin reactivate a deactivated user, and rejects reactivating an already-active one', async () => {
+    const activated = await request(app.getHttpServer())
+      .post(`/api/v1/users/${createdUserId}/activate`)
+      .set('Authorization', `Bearer ${adminAccessToken}`)
+      .expect(201);
+    const body = activated.body as UserBody;
+    expect(body.isActive).toBe(true);
+    expect(body.deactivatedAt).toBeNull();
+
+    await request(app.getHttpServer())
+      .post(`/api/v1/users/${createdUserId}/activate`)
+      .set('Authorization', `Bearer ${adminAccessToken}`)
+      .expect(409);
+  });
 });