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/models/mongo/Dropzone.js

@@ -26,7 +26,9 @@ DropzoneSchema.methods.toJSONFor = function(user){
         oaci: this.oaci,
         createdAt: this.createdAt,
         updatedAt: this.updatedAt,
-        author: this.author.toProfileJSONFor(user)
+        author: this.author && typeof this.author.toProfileJSONFor === 'function'
+            ? this.author.toProfileJSONFor(user)
+            : (user && typeof user.toProfileJSONFor === 'function' ? user.toProfileJSONFor() : this.author ?? null)
     };
 };