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/database/mysql.js

@@ -16,6 +16,7 @@ const TagModel = require('./models/mysql/Tag');
 const TagListModel = require('./models/mysql/TagList');
 const UserModel = require('./models/mysql/User');
 const UserRoleXrefModel = require('./models/mysql/UserRoleXref');
+const SkydiverProfileModel = require('./models/mysql/SkydiverProfile');
 
 const DB = {
   sequelize,
@@ -35,6 +36,7 @@ const DB = {
   Tag: TagModel(sequelize, DataTypes),
   TagList: TagListModel(sequelize, DataTypes),
   UserRoleXref: UserRoleXrefModel(sequelize, DataTypes),
+  SkydiverProfile: SkydiverProfileModel(),
 };
 
 module.exports = DB;