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/library/scan_queue.go

@@ -26,16 +26,19 @@ func (l *Library) ScanLibrary(id int64) error {
 	}
 
 	l.mu.Lock()
-	defer l.mu.Unlock()
 
 	// Silent dedup: already scanning this library.
 	if l.currentScanLibraryID == id {
+		l.mu.Unlock()
+
 		return nil
 	}
 
 	// Silent dedup: already queued.
 	for _, entry := range l.scanQueue {
 		if entry.libraryID == id {
+			l.mu.Unlock()
+
 			return nil
 		}
 	}
@@ -50,6 +53,7 @@ func (l *Library) ScanLibrary(id int64) error {
 		l.scanActive = true
 		l.currentScanLibraryID = entry.libraryID
 		l.currentScanLibraryName = entry.libraryName
+		l.mu.Unlock()
 
 		go l.startScan(entry)
 
@@ -58,13 +62,19 @@ func (l *Library) ScanLibrary(id int64) error {
 
 	// A scan is already running — queue this library.
 	l.scanQueue = append(l.scanQueue, entry)
+	queueLength := len(l.scanQueue)
+	l.mu.Unlock()
 
 	runtime.EventsEmit(l.ctx, events.LibraryScanQueued, map[string]any{
 		"libraryId":   lib.ID,
 		"libraryName": lib.Name,
-		"queueLength": len(l.scanQueue),
+		"queueLength": queueLength,
 	})
 
+	// Registering the queued job takes l.mu again, so it must happen
+	// after the unlock above.
+	l.registerQueuedScanJob(entry)
+
 	return nil
 }
 
@@ -127,6 +137,20 @@ func (l *Library) SoftScanAllLibraries() error {
 	}
 
 	for _, lib := range libs {
+		// A scan the user paused stays paused across restarts — the
+		// soft scan must not quietly start it again behind their back.
+		// RestorePausedScans has already surfaced it in the jobs panel
+		// with a resume button.
+		if l.isScanPausedPersistently(lib.ID) {
+			l.logger.Info(
+				"soft scan: library scan is paused, skipping",
+				"libraryID", lib.ID,
+				"libraryName", lib.Name,
+			)
+
+			continue
+		}
+
 		dbCount, countErr := l.db.Queries.CountAudioFilesByLibrary(
 			l.ctx, lib.ID,
 		)