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>

frontend/src/components/sidebar/app-sidebar.ts

@@ -5,7 +5,7 @@ import { designTokens } from '../../styles/tokens.css';
 
 import type { DragActiveDetail } from '@utils/drag-controller';
 
-type View = 'home' | 'playlists' | 'artists' | 'genres' | 'albums' | 'tracks' | 'explore' | 'autotag' | 'settings';
+type View = 'home' | 'playlists' | 'artists' | 'genres' | 'albums' | 'tracks' | 'explore' | 'autotag' | 'jobs' | 'settings';
 
 interface NavItem {
     id: View;
@@ -150,6 +150,7 @@ export class AppSidebar extends LitElement {
         { id: 'tracks', label: 'Tracks', icon: 'music' },
         { id: 'explore', label: 'Explore', icon: 'globe' },
         { id: 'autotag', label: 'Autotag', icon: 'tag' },
+        { id: 'jobs', label: 'Jobs', icon: 'list-check' },
         { id: 'settings', label: 'Settings', icon: 'gear' },
     ];