Build subscription proration calculator with transparency

[Smartdevs17]

Context

Proration needs transparency and calculation tools.

Current Limitation

Proration is opaque.

Expected Outcome

• Proration calculator
• Transparent proration display
• Proration analytics
• Proration documentation

Acceptance Criteria

• Proration calculator
• Transparent proration display
• Proration analytics
• Proration documentation
• Proration configuration
• Proration API
• Proration testing