Add a central job registry that library scans and search index builds
report into, so background work is visible instead of buried in the
settings page.
- backend/jobs: registry with per-job ring-buffer logs, capability-driven
controls, and one coalesced JobsChanged snapshot at 4Hz
- pause survives restart via a job_state table; a paused scan is adopted
back on launch and skipped by the soft scan
- top-bar indicator, popover, details drawer and a Jobs page replacing
the config page's scan UI; per-library start/stop retained
- scan timing breakdown moves into the job log, Full rescan to the Jobs
page; delete the orphaned library-manager component
Also add cmd/indexbuild and cmd/indexexport so the explore index can be
built once centrally rather than by every install, which today streams
~205GB from the ListenBrainz spark dump on first run. indexbuild picks
build/refresh/rebuild from index state; the Gitea workflow runs it on
push, weekly, or manually and publishes only when content changed.
fresh-install no longer defaults YJ_HOME under /tmp: it is tmpfs on most
distros, and the import needs ~6GB of real disk.
Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>