feat(cluster): "already worked only on the same slot" option

New DX Cluster setting (Settings → DX Cluster). Off (today's behaviour) a call
worked anywhere reads as already worked; on, the WORKED-call flag needs the same
band AND mode. It folds through the digital-mode grouping (Settings → General):
grouped, a 20m FT8 contact also marks a 20m FT4 spot worked; ungrouped they are
separate slots. Backend: qso.WorkedCallSlotKeys builds CALL|band and
CALL|band|mode(grouped) keys; ClusterSpotStatuses uses them when the option is on
(a spot with no inferable mode falls back to same-band).

internal/qso/qso.go

@@ -2163,6 +2163,36 @@ func (r *Repo) WorkedCallBandModeKeys(ctx context.Context) (map[string]struct{},
 	return out, rows.Err()
 }
 
+// WorkedCallSlotKeys backs the cluster "already worked only on the same slot"
+// option. It returns each worked slot keyed BOTH as "CALL|band" (mode-agnostic,
+// for a spot whose mode couldn't be inferred) and "CALL|band|MODE" with the mode
+// run through normMode — so when digital grouping is on, FT8/FT4/RTTY fold into
+// one "DIG" and a call worked on 20m FT8 also matches a 20m FT4 spot. Call is
+// upper-cased and band lower-cased to match the spot keys ClusterSpotStatuses
+// builds. normMode may be nil (no grouping).
+func (r *Repo) WorkedCallSlotKeys(ctx context.Context, normMode func(string) string) (map[string]struct{}, error) {
+	rows, err := r.db.QueryContext(ctx,
+		`SELECT upper(callsign), lower(band), upper(mode) FROM qso
+		 WHERE callsign != '' AND band IS NOT NULL AND band != '' AND mode IS NOT NULL AND mode != ''`)
+	if err != nil {
+		return nil, err
+	}
+	defer rows.Close()
+	out := make(map[string]struct{}, 4096)
+	for rows.Next() {
+		var c, b, m string
+		if err := rows.Scan(&c, &b, &m); err != nil {
+			return nil, err
+		}
+		if normMode != nil {
+			m = normMode(m)
+		}
+		out[c+"|"+b] = struct{}{}
+		out[c+"|"+b+"|"+m] = struct{}{}
+	}
+	return out, rows.Err()
+}
+
 // WorkedPOTARefs returns the set of POTA park references already worked
 // (upper-cased). A QSO's pota_ref may hold several comma-separated parks
 // (an n-fer); each is added separately.