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"

backend/tests/test_layer_enable_refresh.py

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+"""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()

backend/tests/test_perf_safe_optimizations.py

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+"""Regression tests for UX-safe performance optimizations."""
+from __future__ import annotations
+
+import inspect
+
+
+def test_slow_tier_skips_duplicate_time_critical_fetchers():
+    """Weather + Ukraine alerts have dedicated scheduler jobs — not slow tier."""
+    from services import data_fetcher
+
+    source = inspect.getsource(data_fetcher.update_slow_data)
+    slow_block = source.split("_run_tasks(\"slow-tier\"", 1)[0]
+    assert "fetch_weather_alerts" not in slow_block
+    assert "fetch_ukraine_air_raid_alerts" not in slow_block
+
+
+def test_slow_tier_gates_correlation_engine_on_active_layer():
+    from services import data_fetcher
+
+    source = inspect.getsource(data_fetcher.update_slow_data)
+    assert 'is_any_active("correlations")' in source
+
+
+def test_health_uses_subset_refs_not_full_deepcopy():
+    from routers import health as health_router
+
+    source = inspect.getsource(health_router.health_check)
+    assert "_health_data_snapshot()" in source
+    assert "get_latest_data()" not in source
+
+    snap_source = inspect.getsource(health_router._health_data_snapshot)
+    assert "get_latest_data_subset_refs" in snap_source
+    assert "deepcopy" not in snap_source
+
+
+def test_active_layers_defaults_match_dashboard_first_paint():
+    """Backend must not prefetch layers the dashboard starts with disabled."""
+    from services.fetchers import _store
+
+    off_by_default = {
+        "cctv": False,
+        "firms": False,
+        "datacenters": False,
+        "power_plants": False,
+        "psk_reporter": False,
+        "viirs_nightlights": False,
+        "crowdthreat": False,
+        "gt_risk": False,
+    }
+    for key, expected in off_by_default.items():
+        assert _store.active_layers.get(key) is expected, key
+
+
+def test_layer_enable_refresh_covers_cold_toggle_layers():
+    from services import layer_enable_refresh
+
+    source = inspect.getsource(layer_enable_refresh.refresh_newly_enabled_layers)
+    for key in ("cctv", "firms", "power_plants", "psk_reporter", "datacenters"):
+        assert f'"{key}"' in source or f"'{key}'" in source