feat(api): Add deployment rollback capability

[pparage]

Context

When a deployment step fails, there is no way to automatically undo previously completed steps. Failed deployments leave partial infrastructure behind.

Requirements

• On step failure, generate rollback plan from completed steps (reverse order)
• Rollback actions: delete created VMs, remove added network interfaces, delete created bridges, remove firewall rules
• Store rollback plan in deployment record for manual trigger
• Optional: automatic rollback mode vs manual trigger

Dependencies

Requires persistence layer (B1) for tracking completed steps.

Priority

Nice-to-have — Phase 6. Blocks: U11 (rollback UI).

References

• Gap analysis: gap-analysis.md issue B15