fix(skydive): migrate author field from ObjectId to UUID string

- Register SkydiverProfile model in mysql.js (fixes startup crash on association)
- Replace $toObjectId with direct string match in all 12 aggregation queries
- Guard toJSONFor on all Mongo models to fall back to MySQL user.toProfileJSONFor()
  when author is a UUID string (restores username/image in API responses)
- Pass user to toJSONFor() everywhere instead of null
- Remove dead .populate('author') calls (author: String has no ref)
- Fix ownership check in /last route: compare author string to req.payload.id
  instead of the now-absent author.username property

src/routes/api/skydive/applications.routes.js

@@ -10,7 +10,6 @@ mailer.setApiKey(process.env.SENDGRID_API_KEY);
 // Preload application objects on routes with ':application'
 router.param('application', function (req, res, next, slug) {
     Application.findOne({ slug: slug })
-        .populate('author')
         .then(function (application) {
             if (!application) {
                 return res.sendStatus(404);
@@ -52,7 +51,6 @@ router.get('/', auth.required, function (req, res, next) {
                 .skip(Number(offset))
                 .limit(Number(limit))
                 .sort({ createdAt: 'desc' })
-                .populate('author')
                 .exec(),
             Application.countDocuments(query).exec(),
             req.payload ? UserService.getUserById(req.payload.id) : null
@@ -117,8 +115,7 @@ router.post('/', auth.required, function (req, res, next) {
  */
 router.get('/:application', auth.required, function (req, res, next) {
     Promise.all([
-        req.payload ? UserService.getUserById(req.payload.id) : null,
-        req.application.populate('author')
+        req.payload ? UserService.getUserById(req.payload.id) : null
     ]).then(function (results) {
         let user = results[0];
         return res.json({ application: req.application.toJSONFor(user) });