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/library/Library.d.ts

@@ -3,6 +3,7 @@
 import {sqlcgen} from '../models';
 import {library} from '../models';
 import {context} from '../models';
+import {jobs} from '../models';
 
 export function AcquirePipelineLock():Promise<void>;
 
@@ -66,6 +67,8 @@ export function RemoveLibrary(arg1:number):Promise<library.RemovalSummary>;
 
 export function RenameLibrary(arg1:number,arg2:string):Promise<void>;
 
+export function RestorePausedScans():Promise<void>;
+
 export function ResumeScan():Promise<void>;
 
 export function ScanAllLibraries():Promise<void>;
@@ -78,6 +81,8 @@ export function SearchTracksByLibrary(arg1:string,arg2:number):Promise<Array<lib
 
 export function SetContext(arg1:context.Context):Promise<void>;
 
+export function SetJobRegistry(arg1:jobs.Registry):Promise<void>;
+
 export function SetRemovalHooks(arg1:library.RemovalHooks):Promise<void>;
 
 export function SetRescanHooks(arg1:library.RescanHooks):Promise<void>;

frontend/wailsjs/go/library/Library.js

@@ -126,6 +126,10 @@ export function RenameLibrary(arg1, arg2) {
   return window['go']['library']['Library']['RenameLibrary'](arg1, arg2);
 }
 
+export function RestorePausedScans() {
+  return window['go']['library']['Library']['RestorePausedScans']();
+}
+
 export function ResumeScan() {
   return window['go']['library']['Library']['ResumeScan']();
 }
@@ -150,6 +154,10 @@ export function SetContext(arg1) {
   return window['go']['library']['Library']['SetContext'](arg1);
 }
 
+export function SetJobRegistry(arg1) {
+  return window['go']['library']['Library']['SetJobRegistry'](arg1);
+}
+
 export function SetRemovalHooks(arg1) {
   return window['go']['library']['Library']['SetRemovalHooks'](arg1);
 }