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`
Sets up preview deploys etc. for bemo worker.
There's enough going on here that I wanted to make it its own PR. I'll
rework david's spike on top of it once it's landed.
### Change Type
- [x] `internal` — Does not affect user-facing stuff
- [x] `chore` — Updating dependencies, other boring stuff
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Co-authored-by: David Sheldrick <d.j.sheldrick@gmail.com>
