feat(api): Add VM configuration modification endpoints

[pparage]

Context

The API has config getters (CPU, RAM, CDROM, full config) but only one setter (vm_set_tag). Users cannot modify VM resources after creation — they must teardown and redeploy.

Requirements

• PUT /v0/admin/proxmox/vms/{vm_id}/config/cpu — Set CPU cores/sockets
• PUT /v0/admin/proxmox/vms/{vm_id}/config/ram — Set memory allocation
• PUT /v0/admin/proxmox/vms/{vm_id}/config/disk — Resize disk (extend only)
• PUT /v0/admin/proxmox/vms/{vm_id}/config/network — Modify network interface settings
• May require new proxmox_controller actions or direct Proxmox API calls (qm set)
• Handle online vs offline resize (some changes require VM stop)

Priority

Critical (MVP) — Phase 5. Blocks: U5 (post-deployment editing in UI).

References

• Gap analysis: gap-analysis.md issue B5