Feature/prorated plans

[krakenhavoc]

This pull request introduces a new paid plan upgrade flow, including backend API endpoints, service logic, validation, and frontend integration for previewing and executing subscription upgrades. The implementation covers Stripe payment handling, proration calculation, and UI changes for a seamless upgrade experience.

Backend: Plan upgrade APIs and service logic

• Added previewUpgrade and upgradeSubscription methods to BillingService, including plan validation, price difference calculation, Stripe integration, and immediate charging for upgrades.
• Introduced new endpoints in BillingController for /upgrade/preview and /upgrade, with authentication, rate limiting, and proper API documentation.
• Implemented UpgradePlanDto for upgrade requests, validating allowed plans.
• Synced plan from Stripe price ID during subscription updates, ensuring accurate plan tracking.

Backend: Test coverage and Stripe mock updates

• Added comprehensive tests for upgrade preview and upgrade flows, including error cases and Stripe interactions.
• Updated Stripe mocks to support new endpoints and actions for upgrade scenarios. [1] [2]
• Added controller tests for new endpoints. [1] [2]

Frontend: Upgrade flow integration

• Integrated upgrade preview and confirmation modal in Billing.tsx, including error handling, amount formatting, and plan metadata. [1] [2] [3] [4]

Backend: Plan upgrade APIs and service logic

• Added previewUpgrade and upgradeSubscription methods to BillingService for validating upgrades, calculating price differences, and handling Stripe payments and plan changes.
• Introduced /upgrade/preview and /upgrade endpoints in BillingController with authentication, rate limiting, and API docs.
• Implemented UpgradePlanDto for validating upgrade requests.
• Synced plan from Stripe price ID during subscription updates for accurate plan tracking.

Backend: Test coverage and Stripe mock updates

• Added tests for upgrade preview and upgrade flows, including error cases and Stripe mock updates. [1] [2] [3]
• Added controller tests for new endpoints. [1] [2]

Frontend: Upgrade flow integration

• Integrated upgrade preview and confirmation modal in Billing.tsx, with error handling, amount formatting, and plan metadata. [1] [2] [3] [4]