Implement position transfer mechanism for account migrations

[abayomicornelius]

Problem

No way to transfer positions between accounts. Users stuck if they want to migrate.

Why it matters

Account migration is important for key rotation and wallet upgrades.

Suggested fix

Add transfer_position function with proper auth checks and fee handling.

Acceptance criteria

• Add position transfer function
• Require auth from both parties
• Transfer all collateral and debt
• Test transfer scenarios