perf: UX-safe fetch trimming and instant layer-enable refresh.

Drop duplicate slow-tier weather/ukraine jobs, gate correlations when off, slim health probes, keyed layer-panel subscriptions, align backend layer defaults with the dashboard, and fetch CCTV/FIRMS/PSK/etc. synchronously on enable so toggles stay responsive without background prefetch waste.

backend/tests/test_layer_enable_integration.py

@@ -0,0 +1,18 @@
+"""Integration: layer enable triggers immediate data availability."""
+from __future__ import annotations
+
+from services.fetchers._store import active_layers, latest_data, _data_lock
+
+
+def test_firms_enable_populates_slow_payload(client):
+    with _data_lock:
+        active_layers["firms"] = False
+        latest_data["firms_fires"] = []
+
+    r = client.post("/api/layers", json={"layers": {"firms": True}})
+    assert r.status_code == 200
+
+    slow = client.get("/api/live-data/slow")
+    assert slow.status_code == 200
+    fires = slow.json().get("firms_fires") or []
+    assert len(fires) > 0, "firms layer should populate on enable without waiting for scheduler"