Add CPU architecture (cpu_arch) to VM OS images

[v0l]

Problem

VmHost, VmTemplate, and VmCustomTemplate all carry cpu_arch: CpuArch (x86_64 / arm64), but VmOsImage does not. Nothing prevents an x86_64 cloud image from being provisioned onto an arm64 host (or vice versa) — the image catalog is architecture-blind.

Proposal

• Migration: ALTER TABLE vm_os_image ADD COLUMN cpu_arch SMALLINT UNSIGNED NOT NULL DEFAULT 1 (default = x86_64, correct for all existing images)
• Models: add cpu_arch to VmOsImage (lnvps_db), ApiVmOsImage (user API), and the admin create/update/list models
• Provisioning: filter image selection by host/template arch compatibility so incompatible images cannot be deployed
• User API: expose cpu_arch on GET /api/v1/image (open question: also filter the listing by requested/template arch?)
• Docs: update API_DOCUMENTATION.md, ADMIN_API_ENDPOINTS.md, API_CHANGELOG.md

Notes

• CpuArch enum already exists (Unknown = 0, X86_64 = 1, ARM64 = 2) with FromStr/Display (accepts amd64/aarch64 aliases)
• Most distros publish separate cloud images per arch (e.g. Debian -amd64/-arm64, Ubuntu -amd64/-arm64), so the admin UI can eventually offer both variants of the same distro/version