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/wailsjs/go/explore/Service.js

@@ -2,6 +2,14 @@
 // Cynhyrchwyd y ffeil hon yn awtomatig. PEIDIWCH Â MODIWL
 // This file is automatically generated. DO NOT EDIT
 
+export function AdoptPausedIndexBuild() {
+  return window['go']['explore']['Service']['AdoptPausedIndexBuild']();
+}
+
+export function BackfillLibraryDiscographies() {
+  return window['go']['explore']['Service']['BackfillLibraryDiscographies']();
+}
+
 export function BackfillLibraryLyrics() {
   return window['go']['explore']['Service']['BackfillLibraryLyrics']();
 }
@@ -158,6 +166,10 @@ export function SetContext(arg1) {
   return window['go']['explore']['Service']['SetContext'](arg1);
 }
 
+export function SetJobRegistry(arg1) {
+  return window['go']['explore']['Service']['SetJobRegistry'](arg1);
+}
+
 export function SimilarArtists(arg1) {
   return window['go']['explore']['Service']['SimilarArtists'](arg1);
 }