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.

changelog.json

@@ -4,11 +4,13 @@
     "date": "",
     "en": [
       "Cluster: the grid cache now holds 100,000 callsigns and rotates instead of emptying itself, so locators stop vanishing from the list.",
-      "New option \"Chase new grids\": locators learnt from decodes are kept in their own database, so the cluster shows them from the first second."
+      "New option \"Chase new grids\": locators learnt from your decodes and from PSK Reporter are kept in their own database, with their source.",
+      "Band openings: the PSK Reporter feed is now filtered at the broker, which cuts it from about 83 messages a second to under two."
     ],
     "fr": [
       "Cluster : le cache de locators garde 100 000 indicatifs et tourne au lieu de se vider, les locators ne disparaissent donc plus de la liste.",
-      "Nouvelle option « Chasse aux nouveaux carrés » : les locators appris des décodes sont conservés dans leur propre base, le cluster les affiche donc dès la première seconde."
+      "Nouvelle option « Chasse aux nouveaux carrés » : les locators appris de tes décodes et de PSK Reporter sont conservés dans leur propre base, avec leur source.",
+      "Ouvertures de bande : le flux PSK Reporter est désormais filtré chez le broker, ce qui le fait passer d environ 83 messages par seconde à moins de deux."
     ]
   },
   {