feat(jobs): surface background jobs with progress, logs and controls

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>

backend/events/events.go

@@ -87,6 +87,15 @@ const (
 	AutotagPrefetchFinished = "AutotagPrefetchFinished" // {processed, total}
 )
 
+// Background job events.
+const (
+	// JobsChanged carries a full snapshot of every known background job
+	// (see backend/jobs).  A full snapshot rather than a delta means a
+	// component mounting mid-scan is correct from its first event.
+	// Emitted coalesced, at most every 250ms.
+	JobsChanged = "JobsChanged"
+)
+
 // Explore / search index events.
 const (
 	IndexStatusChanged = "IndexStatusChanged"