feat(bandopen): announce sporadic-E openings on 6, 4 and 2 m

Observation, not prediction, and it needs no new data source: the cluster event
worker already enriches every spot with the great-circle distance and bearing
from the operator's grid, which is exactly what a single-hop Es detection rests
on.

The signature is four or more DISTINCT stations at 500-2400 km inside a 90
degree bearing sector within twelve minutes. Each constraint earns its place:
distinct callsigns because one station spotted by six skimmers is six spots and
one station; the lower bound because a 6 m contact under 500 km is ordinary
tropo and says nothing about the ionosphere; the upper bound because past one
hop the bearing test stops meaning anything; and the sector because a real Es
cloud illuminates a direction, which is what separates an opening from a merely
busy evening.

Fed AFTER the Historical guard in the worker. A SH/DX reply replays a hundred
past spots in a second - precisely the shape of a burst - and would announce an
opening that ended hours ago.

Season LABELS, it never gates. Both hemispheres get a summer peak and a lesser
winter one, and an opening outside those is announced with "unusual for the
season" attached: the rare one is the one an operator must not hear about last.

One announcement per band per opening (45 minute quiet period). An opening runs
for hours and produces hundreds of spots; one alert is information, forty is
noise.

changelog.json

@@ -2,8 +2,12 @@
   {
     "version": "0.24.3",
     "date": "",
-    "en": [],
-    "fr": []
+    "en": [
+      "Band openings: OpsLog now tells you when 6, 4 or 2 m opens. It watches the spots already arriving from your clusters and RBN — several different stations appearing at single-hop range (500–2400 km) in the same bearing sector within a few minutes is the signature of sporadic E, and nothing else looks like it. You get one message per band per opening, naming the sector and the typical distance. An opening outside the usual season is still announced, and flagged as unusual: those are the ones worth knowing about. Nothing to configure — but the quality depends on having a cluster or RBN feed carrying VHF spots, and 2 m openings are often worked without ever being spotted."
+    ],
+    "fr": [
+      "Ouvertures de bande : OpsLog te signale désormais l ouverture du 6, du 4 ou du 2 m. Il surveille les spots qui arrivent déjà de tes clusters et du RBN — plusieurs stations différentes apparaissant à distance de saut simple (500–2400 km) dans le même secteur d azimut en quelques minutes, c est la signature de l Es, et rien d autre n y ressemble. Un message par bande et par ouverture, avec le secteur et la distance typique. Une ouverture hors saison est annoncée quand même, et signalée comme inhabituelle : ce sont celles qu il ne faut surtout pas manquer. Rien à configurer — mais la qualité dépend d avoir un flux cluster ou RBN qui porte des spots VHF, et les ouvertures 2 m sont souvent travaillées sans jamais être spottées."
+    ]
   },
   {
     "version": "0.24.2",