Create a crash-consistent snapshot of an instance or volume

Creates a VM snapshot in the active scope from the VM snapshot creation form. The caller chooses the snapshot source — a whole instance (every attached disk is captured) or a single block volume — plus a snapshot name and an optional description. Snapshots are crash-consistent and the source keeps running while the snapshot is captured; storage is billed per captured GB·month pro-rata hourly. On acceptance the snapshot becomes discoverable in the scoped list in its transitional CREATING state with the captured size pre-allocated and usedGb still 0. Validation: name is required and must be non-empty after trimming (422 VALIDATION_ERROR); sourceKind is required and must be instance or volume, and the matching source field must be present — sourceInstanceId for instance, sourceVolume for volume — else 422; a referenced instance or volume that does not exist or is not accessible in the scope maps to 404 NOT_FOUND; snapshot-storage quota exhaustion maps to 409 QUOTA_EXCEEDED. Async & state: Asynchronous (202). The snapshot resource is returned immediately in CREATING state and transitions to terminal AVAILABLE on successful capture or ERROR on failure (e.g. quiesce failed — disk busy); each transition is emitted as a resource.updated SSE frame with a patch on status (and usedGb/incremental once the capture completes) for the snapshot resource. Errors: 401 unauthenticated; 403 caller lacks contributor/admin/owner on the scope; 404 unknown source instance/volume; 409 QUOTA_EXCEEDED when snapshot storage quota is exhausted; 422 invalid body; 500 internal failure. Idempotency/Concurrency: Idempotency-Key is required; replaying the same key returns the originally created snapshot (same id, current state) without capturing a duplicate.