The /?proxyChunkId= endpoint forwards the caller's headers to the volume
server but never mints a read token, so proxied chunk reads return 401
once jwt.signing.read.key is configured. Generate a fileId-scoped volume
token the same way the direct filer read path does, which fixes
filer.sync, filer.backup, filerProxy mounts, the MQ broker and the upload
gateway in one place.