feat(cluster): feed the locator store from PSK Reporter, filtered at the broker

The store shipped without its main source. Locators came only from this
station's own WSJT-X decodes, which is what the whole MQTT discussion was
about.

PSK Reporter now feeds it through a new OnGrid callback, fired before any
geographic filtering: what the store wants is "which square is this callsign
in", and that is true whoever happened to hear the report.

The subscription filters on the RECEIVER's square, a level the v2 topic
exposes. Measured on the live feed: the four opening bands unfiltered are 83
messages a second, of which roughly one in a hundred survived the NearKm test
that already existed here — the rest was received, TLS-decrypted, JSON-parsed
and discarded. One ring of squares is 0.2 to 1.2 a second.

By square rather than by DXCC, which was the obvious alternative: one country
measured 1.2 messages a second (OH) against 72.5 (K) on a single band, because
a DXCC can be a continent. By square the same measurement is 0.2 to 1.2, so the
load follows distance — what the feed is actually about — and is the same for
every operator.

Grid chasing subscribes with the "+" band wildcard, so one subscription per
square covers every band instead of one per band per square.

The store gains a source column (decode | mqtt), migrated in place on an
existing file.

internal/pskr/topics_test.go

@@ -0,0 +1,46 @@
+package pskr
+
+import (
+	"strings"
+	"testing"
+)
+
+// The receiver square is a level the BROKER can filter on, which is the whole
+// point: measured on the live feed, the four opening bands unfiltered are 83
+// messages a second of which about one in a hundred survives the NearKm test.
+// One ring of squares is under two a second, and the same for every operator —
+// where filtering by DXCC ranged from 1.2 (OH) to 72.5 (K) on one band.
+func TestTopicsFilterOnTheReceiverSquare(t *testing.T) {
+	w := New(Config{Bands: []string{"6m"}, RxGrids: []string{"jn36", "JN37"}})
+	got := w.topics()
+	if len(got) != 2 {
+		t.Fatalf("want one subscription per band × square, got %v", got)
+	}
+	// Level order: band/mode/txcall/rxcall/txgrid/RXGRID/txdxcc/rxdxcc
+	want := "pskr/filter/v2/6m/+/+/+/+/JN36/+/+"
+	if got[0] != want {
+		t.Errorf("topic = %q, want %q", got[0], want)
+	}
+	if !strings.Contains(got[1], "/JN37/") {
+		t.Errorf("square not upper-cased into the topic: %q", got[1])
+	}
+}
+
+// With no squares the old behaviour stands: receive the band and decide here.
+func TestTopicsWithoutSquares(t *testing.T) {
+	w := New(Config{Bands: []string{"10m", "2m"}})
+	got := w.topics()
+	if len(got) != 2 || got[0] != "pskr/filter/v2/10m/#" {
+		t.Errorf("unfiltered topics = %v", got)
+	}
+}
+
+// Grid chasing wants every band. "+" is the MQTT single-level wildcard, so one
+// subscription per square covers the lot instead of one per band per square.
+func TestTopicsAllBands(t *testing.T) {
+	w := New(Config{Bands: []string{"+"}, RxGrids: []string{"JN36"}})
+	got := w.topics()
+	if len(got) != 1 || got[0] != "pskr/filter/v2/+/+/+/+/+/JN36/+/+" {
+		t.Errorf("all-band topic = %v", got)
+	}
+}