[Feature] Prioritized Thumbnail Generation for New Uploads

I have searched the existing feature requests to make sure this is not a duplicate request. • Yes The feature As a user with a large media library, I want new uploads to have their thumbnails generated promptly even when a _full-library thumbnail generation_ job is running, so that new media does not show "Error Loading Image" placeholders. Context I occasionally need to run a _full-library thumbnail generation_ job (if I've changed thumbnail settings, etc). With a library of approximately 5TB and 170,000 images/videos on TrueNAS hardware, this job takes 1-2 weeks, using around 50 concurrent tasks. During this time, new uploads have no thumbnails generated, leading to a poor user experience for me and my family. We actively use Immich, and this problem will become more significant as more family members join the platform _and_ as my library grows. The current lack of task prioritization for new uploads results in a temporary "outage" for thu…