feat(agent): add durable existing-agent credential transitions

backend/app/agent_zones/service.py

@@ -177,6 +177,28 @@ class AgentZoneService:
             site_id=assignment.get("site_id"), agent_zone_id=zone_id
         )
 
+    def configuration(self, agent_id: str) -> dict[str, Any] | None:
+        row = self.repository.get_agent_configuration(agent_id)
+        if not row:
+            return None
+        return {
+            "agent_id": row["agent_id"],
+            "site_id": row["site_id"],
+            "site_name": row["site_name"],
+            "agent_zone_id": row["agent_zone_id"],
+            "agent_zone_name": row["agent_zone_name"],
+            "ip_ranges": [
+                {
+                    "name": row["agent_zone_name"],
+                    "cidr": network,
+                    "enabled": row["agent_zone_enabled"] is True,
+                    "api_port": 4028,
+                    "api3_port": 4433,
+                }
+                for network in row.get("networks") or []
+            ],
+        }
+
     def associate_miners_from_agent(
         self, agent_id: str, miner_ids: list[UUID]
     ) -> MinerZoneAssociationResult: