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.

app.go

@@ -2672,6 +2672,23 @@ func (a *App) AddQSO(q qso.QSO) (id int64, err error) {
 		// and the audio was discarded (recordings silently stopped working). The
 		// snapshot is an in-memory copy; the heavy file encode still runs async.
 		a.saveQSORecording(&q)
+		// Drop the cluster worked-index snapshot BEFORE announcing, so the refresh
+		// the frontend fires on qso:logged rebuilds it and sees this QSO.
+		//
+		// Without this the spot colours never moved after a contact. Logging emitted
+		// the event, the frontend dutifully re-queried every visible spot two
+		// seconds later, and ClusterSpotStatuses answered out of a snapshot built
+		// before the QSO — the same "new band" as before, for ever. Reported on an
+		// E51 and again on a ZD7 that stayed yellow on the band map with the QSO
+		// plainly in the log.
+		//
+		// Deliberately NOT invalidateAwardStats(): that also drops the award
+		// matrices, which are expensive to rebuild on a large log, and a contest run
+		// would pay for it once per QSO. This index is a handful of DISTINCT scans
+		// and it is what the spot colours actually read.
+		a.clusterStatusMu.Lock()
+		a.clusterStatusIdx = nil
+		a.clusterStatusMu.Unlock()
 		// Announce the log RIGHT AWAY so the grid/UI refresh at once and the entry
 		// form clears immediately — the operator is not made to wait on the DB.
 		wruntime.EventsEmit(a.ctx, "qso:logged", id)