feat(collectors): add ArcGIS landing points and cable-landing relation collectors

- Add ArcGISLandingPointCollector for FeatureServer/1 (landing points)
- Add ArcGISCableLandingRelationCollector for FeatureServer/3 (cable-landing relations)
- Register new collectors in __init__.py
- Fix earth cables.js with cable_id grouping for highlight

backend/app/services/collectors/arcgis_relation.py

@@ -0,0 +1,58 @@
+from typing import Dict, Any, List
+from datetime import datetime
+
+from app.services.collectors.base import BaseCollector
+
+
+class ArcGISCableLandingRelationCollector(BaseCollector):
+    name = "arcgis_cable_landing_relation"
+    priority = "P1"
+    module = "L2"
+    frequency_hours = 168
+    data_type = "cable_landing_relation"
+
+    base_url = "https://services.arcgis.com/6DIQcwlPy8knb6sg/arcgis/rest/services/SubmarineCables/FeatureServer/3/query"
+
+    async def fetch(self) -> List[Dict[str, Any]]:
+        import httpx
+
+        params = {"where": "1=1", "outFields": "*", "returnGeometry": "true", "f": "geojson"}
+
+        async with httpx.AsyncClient(timeout=60.0) as client:
+            response = await client.get(self.base_url, params=params)
+            response.raise_for_status()
+            return self.parse_response(response.json())
+
+    def parse_response(self, data: Dict[str, Any]) -> List[Dict[str, Any]]:
+        result = []
+
+        features = data.get("features", [])
+        for feature in features:
+            props = feature.get("properties", {})
+
+            try:
+                entry = {
+                    "source_id": f"arcgis_relation_{props.get('OBJECTID', props.get('id', ''))}",
+                    "name": f"{props.get('cable_name', 'Unknown')} - {props.get('landing_point_name', 'Unknown')}",
+                    "country": props.get("country", ""),
+                    "city": props.get("landing_point_name", ""),
+                    "latitude": str(props.get("latitude", "")) if props.get("latitude") else "",
+                    "longitude": str(props.get("longitude", "")) if props.get("longitude") else "",
+                    "value": "",
+                    "unit": "",
+                    "metadata": {
+                        "objectid": props.get("OBJECTID"),
+                        "cable_id": props.get("cable_id"),
+                        "cable_name": props.get("cable_name"),
+                        "landing_point_id": props.get("landing_point_id"),
+                        "landing_point_name": props.get("landing_point_name"),
+                        "facility": props.get("facility"),
+                        "status": props.get("status"),
+                    },
+                    "reference_date": datetime.utcnow().strftime("%Y-%m-%d"),
+                }
+                result.append(entry)
+            except (ValueError, TypeError, KeyError):
+                continue
+
+        return result