uniqueMediaKey checked Exists and the write followed without a
precondition, so two uploads of the same name (clipboard images are all
image.png) could both pick the free key and the second replaced the
first. StoreMedia now reserves the chosen key in-process while it
writes, and writes with IfNotExist; a key another process stored after
the check moves the upload on to the next free name.
- classes.StoreMedia holds the media upload route's storage: extension
allowlist, 10240 KiB cap, /journal/<folder>/<name> keys with the unique
suffix, bucket write and public URL
- UploadMedia keeps authentication, permissions, the folder and form checks
and answers with the same 201 body and messages
- uniqueMediaKey fails instead of overwriting when every numbered name is
taken; the media path tests move to classes
