perf: lightweight post-scan indexing — only index new artists

After a library scan completes, OnAllScansComplete now calls
IndexNewArtists() instead of StartIndexBuild(). This skips the
full tier pipeline (sitewide top artists, similar artists, freshness
checks) and only indexes library artists whose MBIDs are not yet
in the search index.

Flow after scan:
1. Query indexed artist MBIDs (fast, in-memory set)
2. Query library artist MBIDs
3. Diff → only new artists
4. Fetch discographies + images for new artists only

The full StartIndexBuild() still runs on initial launch (when
SoftScanAllLibraries finds no work to do) to handle the tier
pipeline with freshness-based refresh. But adding 5 new albums
to your library no longer triggers a 60-minute index rebuild.

backend/explore/explore.go

@@ -72,6 +72,12 @@ func (e *Service) StartIndexBuild() {
 	e.index.StartBuild(e.ctx)
 }
 
+// IndexNewArtists indexes only library artists not yet in the search
+// index. Lightweight post-scan path — skips the full tier machinery.
+func (e *Service) IndexNewArtists() {
+	e.index.IndexNewArtists(e.ctx)
+}
+
 // StopIndexBuild cancels the background search index build.
 // Call before a full rescan to free the DB for the scan.
 func (e *Service) StopIndexBuild() {