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/jobs/service.go

@@ -0,0 +1,50 @@
+package jobs
+
+// Service is the Wails-bound facade over the registry.  It deliberately
+// exposes only the read and control surface — producers get a *Handle
+// through the registry instead, so the frontend cannot invent jobs.
+type Service struct {
+	reg *Registry
+}
+
+// NewService wraps a registry for frontend binding.
+func NewService(reg *Registry) *Service {
+	return &Service{reg: reg}
+}
+
+// GetJobs returns every known job — active first by registration order,
+// including recently finished ones so the panel can show outcomes.
+func (s *Service) GetJobs() []Job {
+	return s.reg.Snapshot()
+}
+
+// GetJobLog returns the retained log tail for one job.
+func (s *Service) GetJobLog(id string) []LogEntry {
+	return s.reg.Logs(id)
+}
+
+// PauseJob asks the owning subsystem to pause a job.  Returns
+// immediately; the job reports "paused" once it actually stops.
+func (s *Service) PauseJob(id string) {
+	s.reg.Pause(id)
+}
+
+// ResumeJob continues a paused job.
+func (s *Service) ResumeJob(id string) {
+	s.reg.Resume(id)
+}
+
+// CancelJob abandons a job.
+func (s *Service) CancelJob(id string) {
+	s.reg.Cancel(id)
+}
+
+// DismissJob removes a single finished job from the list.
+func (s *Service) DismissJob(id string) {
+	s.reg.Remove(id)
+}
+
+// ClearFinishedJobs removes every terminal job from the list.
+func (s *Service) ClearFinishedJobs() {
+	s.reg.ClearFinished()
+}