Adds an assets server to the demo worker, and reworks the existing asset
server to use the same code. There are a few simplifications to the code
due to some DX improvements working with R2 and caches. I also removed
the `HEAD` request from the assets server: i took a look at our logs and
it's not actually used at all.
This also fixes an issue where users could overwrite the contents of the
asset uploads bucket.
### Change type
- [x] `other`
This PR moves the tldraw.com app into the public repo.
### Change Type
- [x] `internal` — Any other changes that don't affect the published
package[^2]
---------
Co-authored-by: Dan Groshev <git@dgroshev.com>
Co-authored-by: alex <alex@dytry.ch>
