fix: process images for resizing more reliably

[r1tsuu]

Before, we did image resizing in parallel. Meaning if you have 10 different sizes, the machine would process all those at the same time, which potentially can lead to issues with memory, increased CPU usage and unexpected crashes, as the image resizing process is expensive unless you offload it to a different server, which we don't support.

This PR changes to do that sequentially, which could be slower, but more reliable overall which is more important.

Additionally, modifies this call:

if (await fileExists(imagePath)) {
  try {
    fs.unlinkSync(imagePath)
  } catch {
    // Ignore unlink errors
  }
}

to

void fileExists(imagePath).then((value) => {
  if (value) {
    void fs.promises.unlink(imagePath)
  }
})

As we don't really need to await it or block the thread with unlinkSync.

Fixes #8592