revisit background jobs

[Spamercz]

is that up to date with recent rewrite?
Is everything monitored correctly and working?
Is something obsolete?
Is something missing?

[Spamercz]

Audited end to end; the findings became two commits on Spameri/MediaLib.

Obsolete, now gone: the ExtractChapters and ScanAudioTracks job classes (nothing dispatched either), the thumbnails/discovery/chapters/pipeline/audio_tracks job types (that work lives on the agent now), the queue-downloads supervisord workers (two processes idling on a queue no job uses), a no-op marker trait, and PLAN.md's claim that Horizon runs the queues.

Monitoring, now honest: a webshare download's jobs-page entry used to be marked completed the moment it was handed to the machine — it now follows the machine's own progress reports (with downloaded bytes, so speed is finally visible on the Downloads page) and only closes when the machine reports done or failed. Inventory ingestion — the busiest job on the platform — appears on the jobs page at all now, one entry per scan.

Missing, now added: job_statuses was never pruned and grew by every download and enrichment since install — a nightly jobs:cleanup handles it, and also fails download entries whose agent has been silent for over a day, which previously sat "running" forever.