Fix Cloudflare caching unprocessed images

[mdashlw]

Before you begin

• I understand my contributions may be rejected for any reason

• I understand my contributions are for the benefit of Derpibooru and/or the Philomena software

• I understand my contributions are licensed under the GNU AGPLv3

• [x] I understand all of the above

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This PR attempts to fix an issue where Cloudflare would cache the original unprocessed image indefinitely by purging cache after a processor asks to replace the original full.ext file.

$ curl -Is https://derpicdn.net/img/download/2024/5/4/3357501.png | grep -i content-length
content-length: 16570965
$ curl -Is https://derpicdn.net/img/2024/5/4/3357501/full.png | grep -i content-length
content-length: 21585362

I cannot really test this fix.