Implement port tendencies, random trends, decay and improvement #432
Description
Model evolving port behaviour over time.
Parent: #430
Summary:
- Ports can shift tendency (buyer/seller) over time.
- Ports can decay or improve based on traffic and events.
- Changes affect stock levels and prices.
Acceptance criteria:
- Background task to update tendencies and quality over time.
- Visible in port inspection/computer.
- Sane defaults that preserve gameplay balance.
See epic for canon doc links.