recalculate size&hash after web-optimising videos

What does this MR do?

THIS IS A MERGE TO STABLE because this is a bug in the current release

recalculate size&hash after web-optimising videos

there is no guarantee that the ffmpeg command leaves the size unchanged, even with -c copy… and the hash definitely changes!

if we don't, we end up with a size in the database that doesn't match the actual file we're serving, so we emit a wrong content-length, and this confuses clients (and reverse proxies)

@Latte_macchiato take a look please?

thanks to Siina for noticing!

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