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/geo/geo.go

@@ -67,3 +67,54 @@ func DistanceBetweenGrids(a, b string) (km float64, ok bool) {
 	}
 	return HaversineKm(lat1, lon1, lat2, lon2), true
 }
+
+// LatLonToGrid returns the 4-character Maidenhead square for a position.
+func LatLonToGrid(lat, lon float64) string {
+	lon = math.Mod(lon+180, 360)
+	if lon < 0 {
+		lon += 360
+	}
+	lat = lat + 90
+	if lat < 0 {
+		lat = 0
+	} else if lat > 180 {
+		lat = 180
+	}
+	return string([]byte{
+		byte('A' + int(lon/20)),
+		byte('A' + int(lat/10)),
+		byte('0' + int(math.Mod(lon, 20)/2)),
+		byte('0' + int(math.Mod(lat, 10)/1)),
+	})
+}
+
+// NeighbourGrids returns the square holding (lat, lon) and the ring of squares
+// around it — 9 squares for ring 1, 25 for ring 2.
+//
+// Used to filter the PSK Reporter feed at the BROKER rather than in OpsLog. A
+// square is about 111 km tall and 150 km wide at mid latitudes, so one ring is
+// roughly the 300 km "around here" the feed already meant, and the traffic that
+// used to be received and discarded is never sent.
+func NeighbourGrids(lat, lon float64, ring int) []string {
+	if ring < 0 {
+		ring = 0
+	}
+	seen := map[string]bool{}
+	out := []string{}
+	for dLat := -ring; dLat <= ring; dLat++ {
+		for dLon := -ring; dLon <= ring; dLon++ {
+			// One square step: 1° of latitude, 2° of longitude.
+			la := lat + float64(dLat)
+			lo := lon + float64(dLon)*2
+			if la > 90 || la < -90 {
+				continue // past a pole there is no square, not a wrapped one
+			}
+			g := LatLonToGrid(la, lo)
+			if !seen[g] {
+				seen[g] = true
+				out = append(out, g)
+			}
+		}
+	}
+	return out
+}