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/services/fetchers/_store.py

@@ -334,15 +334,15 @@ active_layers: dict[str, bool] = {
     "ships_passenger": True,
     "ships_tracked_yachts": True,
     "earthquakes": True,
-    "cctv": True,
+    "cctv": False,
     "ukraine_frontline": True,
     "global_incidents": True,
     "gps_jamming": True,
     "kiwisdr": True,
     "scanners": True,
-    "firms": True,
+    "firms": False,
     "internet_outages": True,
-    "datacenters": True,
+    "datacenters": False,
     "military_bases": True,
     "sigint_meshtastic": True,
     "sigint_aprs": True,
@@ -353,9 +353,9 @@ active_layers: dict[str, bool] = {
     "satnogs": True,
     "tinygs": True,
     "ukraine_alerts": True,
-    "power_plants": True,
+    "power_plants": False,
     "viirs_nightlights": False,
-    "psk_reporter": True,
+    "psk_reporter": False,
     "correlations": True,
     "contradictions": True,
     "uap_sightings": True,