Preview plan estimate

Preview financial estimate for a plan without creating database records.

This endpoint converts a plan with its products into a contract structure and calls the billables service to calculate cost estimates.

Logic:

1. End date is calculated from start_date + plan.duration if not provided
2. If trial_config is present, a trial phase is created first
3. All products use the plan's duration (single phase for all products)
4. Billables service calculates invoice estimates (dry run)

Trial discount handling:

• Percentage discounts override product pricing discounts (if in applies_to_pricings)
• Fixed discounts are added as features pricing at phase level