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/pskr.go

@@ -81,7 +81,16 @@ type Config struct {
 	// OnSpot receives every accepted decode. Called from the MQTT goroutine, so
 	// it must not block: the broker's buffer is what pays for it if it does.
 	OnSpot func(Spot)
-	Logf   func(string, ...any)
+	// OnGrid receives the transmitter of EVERY message, before any geographic
+	// filtering, for the callsign-to-locator store. Same goroutine as OnSpot and
+	// the same rule: do not block.
+	OnGrid func(call, grid string)
+	// RxGrids filters at the BROKER: only reports collected by a receiver in one
+	// of these squares are sent at all. Empty keeps the old behaviour, which was
+	// to receive the world and discard it here — measured at 83 messages a second
+	// for the four opening bands, of which about one in a hundred survived.
+	RxGrids []string
+	Logf    func(string, ...any)
 }
 
 // Watcher owns the MQTT connection and its subscriptions.
@@ -115,6 +124,32 @@ func New(cfg Config) *Watcher {
 	return &Watcher{cfg: cfg}
 }
 
+// topics builds the subscription list.
+//
+// The v2 topic is
+//
+//	pskr/filter/v2/<band>/<mode>/<tx call>/<rx call>/<tx grid>/<rx grid>/<tx dxcc>/<rx dxcc>
+//
+// so the receiver's square is a level the broker can filter on, and a band of
+// "+" means every band. Filtering by RECEIVER square rather than by receiver
+// DXCC is deliberate: measured on 20 m, one country ranged from 1.2 messages a
+// second (OH) to 72.5 (K), because a DXCC can be a continent. By square the
+// same measurement is 0.2 to 1.2 — the load follows distance, which is what the
+// feed is actually about, and it is the same for every operator.
+func (w *Watcher) topics() []string {
+	out := []string{}
+	for _, b := range w.cfg.Bands {
+		if len(w.cfg.RxGrids) == 0 {
+			out = append(out, "pskr/filter/v2/"+b+"/#")
+			continue
+		}
+		for _, g := range w.cfg.RxGrids {
+			out = append(out, "pskr/filter/v2/"+b+"/+/+/+/+/"+strings.ToUpper(g)+"/+/+")
+		}
+	}
+	return out
+}
+
 // Start connects and subscribes. Safe to call when already running.
 func (w *Watcher) Start() error {
 	w.mu.Lock()
@@ -143,10 +178,7 @@ func (w *Watcher) Start() error {
 
 	opts.OnConnect = func(c mqtt.Client) {
 		w.cfg.Logf("pskr: connected to %s", w.cfg.Broker)
-		for _, b := range w.cfg.Bands {
-			// Every mode, every pair of stations, on this band. That firehose IS
-			// the point: the detector's job is to find the shape in it.
-			topic := "pskr/filter/v2/" + b + "/#"
+		for _, topic := range w.topics() {
 			if tok := c.Subscribe(topic, 0, w.handle); tok.Wait() && tok.Error() != nil {
 				w.cfg.Logf("pskr: subscribe %s failed: %v", topic, tok.Error())
 				continue
@@ -206,6 +238,13 @@ func (w *Watcher) handle(_ mqtt.Client, m mqtt.Message) {
 		return
 	}
 
+	// The locator store takes every transmitter, before any of the geography
+	// below. What it wants is "which square is this callsign in", and that is
+	// true whoever happened to hear the report.
+	if w.cfg.OnGrid != nil {
+		w.cfg.OnGrid(call, grid[:4])
+	}
+
 	// THE RECEIVER HAS TO BE NEAR THE OPERATOR. This is the whole difference
 	// between a useful feed and a world map.
 	//