feat(api): Add bulk operations for arbitrary VM lists

[pparage]

Context

Current bulk operations are hardcoded to admin/student/vuln groups via dedicated bundle endpoints. There is no way to perform bulk start/stop/delete on an arbitrary set of VM IDs.

Requirements

• POST /v0/admin/proxmox/vms/bulk/start — Accept { "vm_ids": [100, 101, 102] }
• POST /v0/admin/proxmox/vms/bulk/stop
• POST /v0/admin/proxmox/vms/bulk/pause
• POST /v0/admin/proxmox/vms/bulk/resume
• DELETE /v0/admin/proxmox/vms/bulk/delete
• Return per-VM results (success/failure for each)

Note

The existing mass_start_stop_pause_resume endpoint exists but is also group-based. These new endpoints should accept arbitrary VM ID arrays.

Priority

Important — Phase 5. Blocks: U12 (arbitrary VM grouping in UI).

References

• Gap analysis: gap-analysis.md issue B13