Application was added to models/mysql/index.js but not to mysql.js,
which is the actual file that instantiates all Sequelize models.
DB.Application was undefined, causing the relationships to fail at startup.
- 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