Add dynamically calculated total amount paid to maintainers to OpenGraph image

[vladh]

Closes #313

This OpenGraph image is automatically generated from our latest total tally on every build:

Workflows already tested — OK to merge without babysitting.

Story time

This was trickier than expected:

• We generate an SVG from the calculated amount, but it has to be converted into a PNG, which gets us into trouble, because we need to rasterise the fonts correctly.
• ImageMagick does not support being given specific font files — it uses fontconfig to find the fonts in standard system paths. But we also don't want to just install our fonts into system paths willy-nilly. So I had to create a solution that will install fonts for the user for only a few seconds.
• Inkscape exports SVG markup that renders correctly in Firefox, but incorrectly with the version of imagemagick in the Arch Linux packages, and incorrectly in a different way on the latest imagemagick I could get for GitHub workflows. So I had to spend a bit of time rewriting the SVG by hand to make it as simple as possible, until it rendered correctly everywhere.

But also:

• This was a good opportunity to centralise our member data processing code into memberData/common.ts.
• The ImageMagick installation does not seem to add much time to our GitHub workflows build time.

[chadwhitacre]

2025 🙃