feat(explore): backfill owned artists' discographies offline post-scan

Enrich owned artists whose discography hasn't been fetched yet in a
bounded, resumable background pass so their wider catalogue is searchable
offline right after a scan, instead of only on first artist-page view.

Keyed off the persistent discog_fetched flag via LEFT JOIN, so already-
enriched artists never reappear and the run is a cheap no-op once every
owned artist is covered. Capped at discogBackfillMaxPerRun per run and
routed through discogSF to avoid double-fetching an artist a concurrent
interactive EnsureArtistDiscography is handling. Invoked on both scan
completion (OnStartup) and OnDomReady to resume a capped/interrupted run.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/explore/explore.go

@@ -177,6 +177,15 @@ func (e *Service) PopulateLocalCrossReferencesIfNeeded() {
 	e.index.PopulateLocalCrossReferences()
 }
 
+// BackfillLibraryDiscographies enriches owned artists that have not had
+// their discography fetched yet, in the background.  Idempotent and
+// bounded — the query only returns unenriched artists and each is marked
+// discog_fetched on success, so this is cheap (an empty query) once every
+// owned artist is covered and safe to call on every scan and launch.
+func (e *Service) BackfillLibraryDiscographies() {
+	go e.index.BackfillLibraryDiscographies(e.ctx)
+}
+
 // InvalidateLibrarySync clears the "ready" markers guarding the gated
 // library-sync steps so they re-run on the next launch.  Call after a
 // mutation that changes owned content outside a scan (e.g. removing a