fix(cluster): drop the worked index when a QSO is logged

AddQSO emitted qso:logged but never invalidated the cluster status snapshot.
The frontend did its part - it re-queried every visible spot two seconds later -
and ClusterSpotStatuses answered out of a snapshot built before the contact, so
it returned exactly the same "new band" as before. For ever, until an import, an
edit or a profile switch happened to invalidate it for another reason.

Reported twice: an E51 and then a ZD7 that stayed yellow on the band map with
the QSO plainly in the log. The first report was answered by fixing the
visibility gate, which was a real bug of its own and hid this one.

Confirmed against the operator's MySQL logbook rather than guessed: ZD7BG has
dxcc=250 on all eleven QSOs and 0 of 29579 rows lack a DXCC number, which ruled
out the missing-entity-number theory and pointed here.

Deliberately not invalidateAwardStats(): that also drops the award matrices,
which are expensive on a large log, and a contest run would pay for it once per
QSO. This index is a few DISTINCT scans and it is what the spot colours read.

Also fixes the new worked-grid query, which asked for a column named
"gridsquare". The column is "grid" - gridsquare_ext is a different one - so NEW
GRID could never have worked on either backend. Verified against the real
database: 12516 distinct grid|mode pairs.

internal/qso/qso.go

@@ -3017,8 +3017,10 @@ func IsFilterableExtra(field string) bool   { _, ok := filterableExtras[field];
 // forever, once per subsquare.
 func (r *Repo) WorkedGridKeys(ctx context.Context, normMode func(string) string) (map[string]struct{}, error) {
 	rows, err := r.db.QueryContext(ctx,
-		`SELECT DISTINCT COALESCE(gridsquare,''), COALESCE(mode,'') FROM qso
-		 WHERE gridsquare IS NOT NULL AND gridsquare != ''`)
+		// The column is "grid", not "gridsquare" — that is the ADIF field name, and
+		// gridsquare_ext is a different (six-plus character) column entirely.
+		`SELECT DISTINCT COALESCE(grid,''), COALESCE(mode,'') FROM qso
+		 WHERE grid IS NOT NULL AND grid != ''`)
 	if err != nil {
 		return nil, err
 	}