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.

decodegrids_test.go

@@ -22,7 +22,7 @@ func TestDecodeGridRotationKeepsThePreviousGeneration(t *testing.T) {
 
 	// Fill exactly one generation.
 	for i := 0; i < decodeGridsCap; i++ {
-		a.rememberDecodeGrid(fmt.Sprintf("CALL%06d", i), "JN36")
+		a.rememberDecodeGrid(fmt.Sprintf("CALL%06d", i), "JN36", gridcache.SourceDecode)
 	}
 	if got := a.lookupDecodeGrid("CALL000000"); got != "JN36" {
 		t.Fatalf("first entry lost before any rotation: %q", got)
@@ -32,7 +32,7 @@ func TestDecodeGridRotationKeepsThePreviousGeneration(t *testing.T) {
 	}
 
 	// One more entry rotates.
-	a.rememberDecodeGrid("NEWCALL", "IO91")
+	a.rememberDecodeGrid("NEWCALL", "IO91", gridcache.SourceDecode)
 	if a.decodeGridsOld == nil {
 		t.Fatal("did not rotate at the cap")
 	}
@@ -49,7 +49,7 @@ func TestDecodeGridRotationKeepsThePreviousGeneration(t *testing.T) {
 
 	// A second rotation is what finally retires the oldest half.
 	for i := 0; i < decodeGridsCap; i++ {
-		a.rememberDecodeGrid(fmt.Sprintf("SECOND%06d", i), "KP20")
+		a.rememberDecodeGrid(fmt.Sprintf("SECOND%06d", i), "KP20", gridcache.SourceDecode)
 	}
 	if got := a.lookupDecodeGrid("CALL000000"); got != "" {
 		t.Errorf("the cache is unbounded: %q survived two rotations", got)
@@ -63,12 +63,12 @@ func TestDecodeGridRotationKeepsThePreviousGeneration(t *testing.T) {
 // "f4bpo" would miss a grid learnt as "F4BPO".
 func TestDecodeGridCaseAndBlanks(t *testing.T) {
 	a := &App{}
-	a.rememberDecodeGrid(" f4bpo ", "JN36")
+	a.rememberDecodeGrid(" f4bpo ", "JN36", gridcache.SourceDecode)
 	if got := a.lookupDecodeGrid("F4BPO"); got != "JN36" {
 		t.Errorf("lookup of the upper-case form failed: %q", got)
 	}
-	a.rememberDecodeGrid("", "JN36")
-	a.rememberDecodeGrid("K1ABC", "")
+	a.rememberDecodeGrid("", "JN36", gridcache.SourceDecode)
+	a.rememberDecodeGrid("K1ABC", "", gridcache.SourceDecode)
 	if got := a.lookupDecodeGrid("K1ABC"); got != "" {
 		t.Errorf("stored an empty grid: %q", got)
 	}
@@ -87,7 +87,7 @@ func TestDecodeGridConcurrentAccess(t *testing.T) {
 	go func() {
 		defer wg.Done()
 		for i := 0; i < 20000; i++ {
-			a.rememberDecodeGrid(fmt.Sprintf("W%05d", i), "FN31")
+			a.rememberDecodeGrid(fmt.Sprintf("W%05d", i), "FN31", gridcache.SourceDecode)
 		}
 	}()
 	go func() {
@@ -112,7 +112,7 @@ func TestOnlyChangesAreQueued(t *testing.T) {
 	defer st.Close()
 	a := &App{gridStore: st}
 
-	a.rememberDecodeGrid("F4BPO", "JN36")
+	a.rememberDecodeGrid("F4BPO", "JN36", gridcache.SourceDecode)
 	if n := st.Pending(); n != 1 {
 		t.Fatalf("a new locator queued %d writes, want 1", n)
 	}
@@ -122,14 +122,14 @@ func TestOnlyChangesAreQueued(t *testing.T) {
 
 	// The same report, a hundred times over, is not news.
 	for i := 0; i < 100; i++ {
-		a.rememberDecodeGrid("F4BPO", "JN36")
+		a.rememberDecodeGrid("F4BPO", "JN36", gridcache.SourceDecode)
 	}
 	if n := st.Pending(); n != 0 {
 		t.Errorf("unchanged reports queued %d writes — the batch would carry the whole feed", n)
 	}
 
 	// A station that moved is.
-	a.rememberDecodeGrid("F4BPO", "KP30")
+	a.rememberDecodeGrid("F4BPO", "KP30", gridcache.SourceDecode)
 	if n := st.Pending(); n != 1 {
 		t.Errorf("a changed locator queued %d writes, want 1", n)
 	}
@@ -149,7 +149,7 @@ func TestNoRotationWhilePersisting(t *testing.T) {
 	a := &App{gridStore: st}
 
 	for i := 0; i < decodeGridsCap+10; i++ {
-		a.rememberDecodeGrid(fmt.Sprintf("CALL%06d", i), "JN36")
+		a.rememberDecodeGrid(fmt.Sprintf("CALL%06d", i), "JN36", gridcache.SourceDecode)
 	}
 	if a.decodeGridsOld != nil {
 		t.Error("rotated while a store was attached — locators the database holds would go missing")