B-T2 changed Scanner.Scan to take a progressCb parameter, but the
ScanTrigger interface in internal/server/server.go was missed. The
interface gains the same progressCb arg; the HTTP handler at
handleAdminScan passes nil (fire-and-forget triggers don't observe
partial tallies). The stubScanner in server_test.go gets the new
arg too.
bvandeusen
6e985cf532