fix:[FL-173][修复usage_status验证错误]

将 TempDataset 的 usage_status 从 "active" 改为 "idle"，符合 ElevationDatasetUsageStatus 的 Literal 定义（"idle" 或 "in_use"）。

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Co-authored-by: multica-agent <github@multica.ai>

api/app/services/elevation_file_record_service.py

@@ -731,7 +731,7 @@ def preview_file_record(
                 self.file_path = record.file_path
                 self.resolution_m = record.resolution_m
                 self.status = record.status
-                self.usage_status = "active"
+                self.usage_status = "idle"
                 self.sample_count = record.sample_count
                 self.bbox_min_lon = record.bbox_min_lon
                 self.bbox_max_lon = record.bbox_max_lon