fix(server/m7-scan-progress): forward-fix CI — ScanTrigger interface

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.

internal/server/server_test.go

@@ -247,6 +247,6 @@ func TestRouter_AdminSubtreeNotShadowed(t *testing.T) {
 // route-presence assertions in this file.
 type stubScanner struct{}
 
-func (stubScanner) Scan(_ context.Context) (library.Stats, error) {
+func (stubScanner) Scan(_ context.Context, _ func(library.Stats)) (library.Stats, error) {
 	panic("stubScanner.Scan called from server_test")
 }