Add rental storage management feature

[valkanm]

Closes #593

This PR introduces storage management feature that allows users to reserve storage units and view unit availability. Key functionality includes:

• Define storage units with price and maximum rental months
• Users can create reservations, extend them, and track total paid months
• Integration with bank transactions to mark reservations as paid and generate StoragePayment records
• Admins can view full reservation history per unit
• Role-based permissions: normal users can only have one active or pending reservation, admins have no limits

Testing: unit tests added for reservation creation, payment processing, and extending reservations.