Migrate most usage of FastAPI BackgroundTasks --> ArqRedis queue

[spwoodcock]

• Using background tasks to do things like send emails is fine.
• We also use them to do things like extract file assets from ODM.
• I assume this is for legacy reasons, before arq was introduced.
• Assess where background tasks could be replaced with arq.

Additional context

• It's not ideal that arq recently went in 'low maintenance mode'.
• We probably will need to find a replacement before too long...

[linear]

TECH-950