Fix Proxmox scheduled sync

[ZacharyZcR]

Summary

• serialize resolved Proxmox jump-host arrays before inserting synced guests into SQLite
• keep manual discovery responses normalized without changing their behavior
• add regression coverage for array, stored JSON, and malformed jump-host values

Validation

• npm test -- --run src/backend/tests/database/routes/proxmox-jump-hosts.test.ts src/backend/tests/database/host-repository.test.ts src/backend/tests/database/routes/proxmox-import-auth.test.ts
• npm run lint
• npm run type-check
• npm run build

Fixes Termix-SSH/Support#1196