feat(migration): add target node capacity validation

[Anhydrite]

Problem

When migrating a VM to another node, there is no capacity validation on the target node. The system only checks:

1. Target node is not empty
2. Target node is different from source

Current Behavior

• VM migration proceeds even if target node has no available CPU, RAM, or disk
• Migration may succeed but VM won't start on target due to resource exhaustion
• No error returned to user explaining why migration failed

Expected Behavior

Before migrating, validate target node can accommodate the VM:

1. Query target node's current usage (CPU, RAM, disk)
2. Compare against VM's requested resources
3. Reject with clear error: Target