feat: the library's folders are set in Admin › Library, and changing a folder moves its tracks with it, keeping their plays, likes and playlists (#6129)

Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

internal/api/api.go

@@ -299,6 +299,11 @@ func Mount(r chi.Router, pool *pgxpool.Pool, logger *slog.Logger, events *playev
 				admin.Get("/library/reacquisition", h.handleGetReacquisitionSettings)
 				admin.Put("/library/reacquisition", h.handleUpdateReacquisitionSettings)
 
+				// Where the library lives (#6129). A changed folder moves its
+				// tracks rather than re-adding them.
+				admin.Get("/library/folders", h.handleGetLibraryFolders)
+				admin.Put("/library/folders", h.handlePutLibraryFolders)
+
 				admin.Get("/scan/status", h.handleGetScanStatus)
 				admin.Post("/scan/run", h.handleTriggerScan)
 				// Sits under /library rather than /tracks because what it
@@ -498,11 +503,13 @@ func artSources(s *coverart.SettingsService) catalogue.ArtSources {
 	return s
 }
 
-// scannerRoots are the library folders, or none when the API runs without a
-// scanner (tests).
-func scannerRoots(s *library.Scanner) []string {
-	if s == nil {
-		return nil
+// scannerRoots reads the library folders, or none when the API runs without
+// a scanner (tests).
+func scannerRoots(s *library.Scanner) func() []string {
+	return func() []string {
+		if s == nil {
+			return nil
+		}
+		return s.Roots()
 	}
-	return s.Roots()
 }