Fix Proxmox discovery for sudo users

[ZacharyZcR]

Summary

• run every Proxmox discovery pvesh read through the existing elevated-command path when the host has a stored sudo password
• preserve direct execution for root hosts without sudo credentials
• cover forced elevation and failed-command propagation with focused regression tests

Fixes Termix-SSH/Support#1246

Validation

• npx vitest run --project backend src/backend/tests/database/routes/proxmox-sudo.test.ts src/backend/tests/hosts/metrics/managers/exec-elevated.test.ts
• npx eslint src/backend/database/routes/proxmox.ts src/backend/tests/database/routes/proxmox-sudo.test.ts
• npx prettier --check src/backend/database/routes/proxmox.ts src/backend/tests/database/routes/proxmox-sudo.test.ts
• npx tsc --noEmit
• git diff --check