feat(cluster): configurable spot lifetime

Spots were bounded by COUNT alone, so on a quiet band a two-hour-old spot sat
on the band map looking like something to chase. Settings → DX Cluster now
takes a lifetime: presets at 5/10/15/30/60 minutes, or any value up to twelve
hours, and 0 keeps the old behaviour.

Spots are REMOVED from the shared list rather than hidden at render. The
cluster list, every band map and the counts all read that one array, so pruning
it once is what makes the setting mean the same thing in every view — the
lesson already paid for when the band map ignored the cluster's filters. It
also gives the memory back.

A spot whose timestamp will not parse is never dropped: an unreadable stamp is
a reason to distrust the clock, not to throw away the spot.

Swept every 30 seconds — close enough that a 5-minute setting is honoured, and
invisible next to the spot stream. Clamped in the backend as well as the UI.

changelog.json

@@ -3,10 +3,12 @@
     "version": "0.25.0",
     "date": "",
     "en": [
-      "Cluster: \"Hide worked\" no longer hides a spot that is a new prefix, county, grid or park in an entity already worked."
+      "Cluster: \"Hide worked\" no longer hides a spot that is a new prefix, county, grid or park in an entity already worked.",
+      "DX Cluster: a spot lifetime can be set — 5, 10, 15 minutes or your own value — after which spots leave the list and the band maps."
     ],
     "fr": [
-      "Cluster : « Masquer les contactés » ne masque plus un spot qui est un nouveau préfixe, comté, carré ou parc dans une contrée déjà faite."
+      "Cluster : « Masquer les contactés » ne masque plus un spot qui est un nouveau préfixe, comté, carré ou parc dans une contrée déjà faite.",
+      "Cluster DX : on peut fixer une durée de vie des spots — 5, 10, 15 minutes ou une valeur libre — au-delà de laquelle ils quittent la liste et les band maps."
     ]
   },
   {