Bulk-create assets

Create up to 100 assets in one request — for onboarding a client estate programmatically. Requires a write-scoped key. The plan asset quota is enforced across the whole batch (the request is rejected with 402 if it would exceed the ceiling). Per-asset failures are reported in errors without aborting the batch; valid rows are still created. Returns 201 when at least one asset was created or revived, 207 when every row failed. TD-2480: a row whose hostname belongs to a previously-deleted asset RESTORES that asset (with its check history) and is reported in revived / revived_count, not in created — skipped now means only that an asset you can still see already has that hostname.