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/index.ts

@@ -20,6 +20,9 @@ import '@components/explore-view/explore-view.ts';
 import '@components/explore-artist-details/explore-artist-details.js';
 import '@components/explore-album-details/explore-album-details.js';
 import '@components/autotag-view/autotag-view.ts';
+import '@components/first-run-wizard/first-run-wizard.ts';
+import '@components/jobs/job-indicator.ts';
+import '@components/jobs/jobs-view.ts';
 import '@awesome.me/webawesome/dist/styles/themes/default.css';
 import '@awesome.me/webawesome/dist/components/icon/icon.js';
 import { setBasePath } from '@awesome.me/webawesome/dist/webawesome.js';
@@ -61,6 +64,7 @@ const VIEW_TAGS: Record<string, string> = {
     playlists: 'playlist-view',
     explore: 'explore-view',
     autotag: 'autotag-view',
+    jobs: 'jobs-view',
     settings: 'config-page',
 };