Give option to calculate final payment for cancelled subscriptions using number of sessions before end of notice period

Summary:
Introduce an option to calculate the final payment for cancelled subscriptions based on the number of sessions remaining within the notice period, rather than a pro-rata percentage of time.

Current Behaviour:

• When a subscription is cancelled with a notice period of 1 month or more, the system always generates a final payment.

• This payment is taken on the next billing date (typically the 1st of the month).

• The amount is calculated as a pro-rata percentage of the normal subscription amount, based on the remaining time in the notice period.

  • Example: If cancelled on 10th April, the subscription ends on 10th May.

  • The final payment on 1st May is ~1/3 (10 days’ worth) of the monthly fee.

Requested Behaviour:

• Add a configurable option to calculate the final payment based on the number of scheduled sessions within the notice period instead of time.

• The system should:

  • Identify all sessions occurring between the next billing date and the end of the notice period.

  • Calculate the final payment based only on those sessions.

Example Scenario:

• Cancellation date: 10th April

• Notice period ends: 10th May

• If there are no sessions scheduled in early May (e.g. due to holidays), then:

  • No sessions fall within the final billing period

  • Final payment on 1st May = £0

Expected Outcome:

• Users can choose between:

  1. Time-based pro-rata calculation (current behaviour)

  2. Session-based calculation (new option)

• This provides more accurate billing for businesses with irregular schedules (e.g. school term breaks).