fix: scope FileBrowser DB reset to restore-only path

Normal storage add/remove no longer nukes the FileBrowser database volume.
A .fb-reset flag file is written during restore and consumed on next startup.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

controller/internal/setup/handlers.go

@@ -875,6 +875,11 @@ func (s *Server) restoreFromInfraBackup(ib *report.InfraBackup) {
 			}
 		}
 	}
+
+	// Signal that FileBrowser's database should be reset on next startup.
+	// After restore, the DB has stale source preferences from the initial install.
+	flagPath := filepath.Join(s.dataDir, ".fb-reset")
+	_ = os.WriteFile(flagPath, []byte("restore"), 0644)
 }
 
 // mountDrivesFromBackup mounts drives from the infra backup's disk layout.