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.

frontend/src/lib/i18n.tsx

@@ -270,7 +270,7 @@ const en: Dict = {
   'clu.muteWorkedHint': '(they stay in the list, just quiet — leaves the colour for what is left to do)',
   'clu.slotHighlight': 'Colour the stations not worked on this band and mode',
   'clu.slotHighlightHint': '(by callsign, whatever the entity status says)',
-  'bo.open': 'open', 'bo.liveTip': '{band} is open — {n} stations, ~{km} km, {sector}{season}. Click for the band map.', 'bo.enable': 'Watch for band openings', 'bo.enableHint': '(10, 12, 6, 4 and 2 m. Switching this on adds the two RBN nodes and subscribes to the PSK Reporter feed — the detection needs far more ears than a cluster can give it.)', 'bo.feedUp': 'PSK Reporter feed up — {n} decodes seen', 'bo.feedDown': 'PSK Reporter feed down — needs your station grid, and a moment to connect', 'clu.chaseGrids': 'Chase new grids', 'clu.chaseGridsHint': '(keeps the locators learnt from WSJT-X decodes in their own database, so the cluster shows them from the first second instead of after an hour of listening)', 'clu.workedSameSlot': 'Already worked only on the same slot',
+  'bo.open': 'open', 'bo.liveTip': '{band} is open — {n} stations, ~{km} km, {sector}{season}. Click for the band map.', 'bo.enable': 'Watch for band openings', 'bo.enableHint': '(10, 12, 6, 4 and 2 m. Switching this on adds the two RBN nodes and subscribes to the PSK Reporter feed — the detection needs far more ears than a cluster can give it.)', 'bo.feedUp': 'PSK Reporter feed up — {n} decodes seen', 'bo.feedDown': 'PSK Reporter feed down — needs your station grid, and a moment to connect', 'clu.chaseGrids': 'Chase new grids', 'clu.chaseGridsHint': '(learns locators from your own WSJT-X decodes AND from PSK Reporter, and keeps them in their own database so the cluster shows them from the first second)', 'clu.chaseGridsStat': '{n} locators known — {p} waiting to be written', 'clu.workedSameSlot': 'Already worked only on the same slot',
   'clu.workedSameSlotHint': '— a spot shows "worked" only if you worked that call on the SAME band and mode, not just anywhere. Combines with digital-mode grouping (Settings → General): with it on, a call worked on 20m FT8 also counts as worked for a 20m FT4 spot; with it off, FT8 and FT4 are separate slots.',
   // Backup panel
   'bk.hintMysql': 'On close (once/day) OpsLog snapshots the local SQLite (config) AND exports the shared MySQL log to ADIF — opslog-log-<date>.adi — so your contacts are protected even though they live on the server. Rotation keeps the last N of each.',