#45 Issue 45: [Logic] Fixed-End Date vs. Fixed-Amount Streams


Description:
Currently, streams are amount-based. Add support for "Until Date" streams (e.g., "Stream $1000/mo until Dec 31st") where the total amount is calculated based on the timeframe.
Acceptance Criteria:

[ ] Add StreamType enum.

[ ] Implement math for EndDate calculation during create_grant.
Labels: logic, flexibility