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_refresh.py

@@ -0,0 +1,36 @@
+"""Tests for on-enable layer refresh (Phase 2 UX guardrail)."""
+from __future__ import annotations
+
+from unittest.mock import patch
+
+from services.fetchers._store import active_layers, bump_active_layers_version
+from services.layer_enable_refresh import refresh_newly_enabled_layers, snapshot_active_layers
+
+
+def test_refresh_firms_on_enable_only():
+    before = snapshot_active_layers()
+    active_layers["firms"] = True
+    bump_active_layers_version()
+
+    with (
+        patch("services.fetchers.earth_observation.fetch_firms_fires") as firms,
+        patch("services.fetchers.earth_observation.fetch_firms_country_fires") as country,
+        patch("services.fetchers._store.bump_data_version") as bump,
+    ):
+        refresh_newly_enabled_layers({**before, "firms": False})
+
+    firms.assert_called_once()
+    country.assert_called_once()
+    bump.assert_called_once()
+
+    active_layers["firms"] = before.get("firms", False)
+
+
+def test_refresh_skips_when_layer_stays_off():
+    before = {**snapshot_active_layers(), "cctv": False}
+    active_layers["cctv"] = False
+
+    with patch("services.fetchers.infrastructure.fetch_cctv") as fetch_cctv:
+        refresh_newly_enabled_layers(before)
+
+    fetch_cctv.assert_not_called()