feat(ecau): extract Soundcloud backdrop images

[ROpdebee]

While fixing #526 I noticed that the Soundcloud tracks in that set have backdrop images, so let's extract those too. Also included is a fix for #526. Not sure if this problem has always existed or was caused by a Soundcloud change, because we didn't have tests for sets with >5 tracks.

[codecov[bot]]

Codecov Report

Merging #543 (b92d153) into main (ecd5200) will decrease coverage by 0.62%. The diff coverage is 80.43%.

@@            Coverage Diff             @@
##             main     #543      +/-   ##
==========================================
- Coverage   99.17%   98.54%   -0.63%     
==========================================
  Files          57       57              
  Lines        1339     1378      +39     
  Branches      212      219       +7     
==========================================
+ Hits         1328     1358      +30     
- Misses          8       13       +5     
- Partials        3        7       +4     

Impacted Files	Coverage Δ
...enhanced_cover_art_uploads/providers/soundcloud.ts	86.36% <75.67%> (-13.64%)	:arrow_down:
...rc/mb_enhanced_cover_art_uploads/providers/base.ts	96.72% <100.00%> (+0.29%)	:arrow_up:
src/lib/util/async.ts	100.00% <0.00%> (ø)
src/lib/util/observable.ts	100.00% <0.00%> (ø)

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[github-actions[bot]]

:rocket: Released 1 new userscript version(s):

• mb_enhanced_cover_art_uploads 2022.8.5 in 9c999635c636c1ed68d9a3dad1b87c6a5151fa16