[FEATURE] Resource previews script parallelisation

[VeckoTheGecko]

Describe your suggested improvement
The get_og_previews.py script is currently run sequentially for each resource. Running this in parallel would result in better build speeds for the website.

Describe the benefit
Build speed

Would you be capable/willing to implement the improvement?
Perhaps. Happy for a community member to claim this.

Additional comments

[Lyrete]

Pretty bored so will probably start looking at this later today or tomorrow :) Will ditch urllib for httpx and should be relatively simple to get going after that.

Can rescale too but as mentioned in #373 I think it's probably nice to just leave this for Svelte to deal with (or rather more futureproof if there's ever a change in the card format). Could however ditch previews that are too small to use pre-scaling?