feat(cluster): persist learnt locators behind a "Chase new grids" option

The callsign->grid map died with the process. Every restart began with an empty
Locator column that took an hour of listening to refill, and everything learnt
the day before was thrown away.

internal/gridcache is its own SQLite file in the data directory, not a table in
the settings database: that one sits wherever the operator chose to put it,
often a synchronised folder, and a store that rewrites itself every minute has
no business there. Deleting the file costs a few days of listening and nothing
else.

Callsign is the primary key and the newest report wins — operators move, go
portable, go on expedition, and a stale locator is worse than none for grid
chasing because it reads as a square already worked. Entries not seen in two
years are pruned at open: that is the one way this cache can be actively wrong
rather than merely empty, and it is what bounds a store that would otherwise
only grow.

Only CHANGES are queued. The feeds repeat themselves, so writing every report
would put the whole stream in the batch instead of the news in it. Batches
flush on a timer and on shutdown — a restart is exactly when the cache is worth
the most.

Rotation is switched off while the store is attached: the cap would discard
callsigns the database still holds and the lookup would then miss what we know.
Age in the store is the bound instead.

changelog.json

@@ -3,10 +3,12 @@
     "version": "0.24.8",
     "date": "",
     "en": [
-      "Cluster: the grid cache now holds 100,000 callsigns and rotates instead of emptying itself, so locators stop vanishing from the list."
+      "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."
     ],
     "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."
+      "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."
     ]
   },
   {