feat: Add Subscription Items API endpoints

[Abhishek-singh88]

Resolves #159

Description

This PR adds public API endpoints for Subscription Items to match Stripe's API. Previously, subscription items could only be managed implicitly by updating the parent subscription. This provides developers with dedicated endpoints to manage items directly.

Files Changed:

• apps/api/src/modules/Subscription.ts: Added public CRUD methods (CreateItem, GetItem, UpdateItem, DeleteItem, ListItems) for Subscription Items. These hook into the existing database models, reuse internal logic, and emit necessary webhook events (like customer.subscription.updated). Modified RequireSubscription to optionally validate the platform account.
• apps/api/src/routes/subscriptionItems.routes.ts: Created new Express routes to handle the endpoints. Validates incoming requests using the existing Zod schemas (CreateSubscriptionItemSchema, UpdateSubscriptionItemSchema, DeleteSubscriptionItemSchema).
• apps/api/src/routes/index.ts: Mounted the new subscriptionItems router to the /v1/subscription_items path.
• apps/api/src/__tests__/SubscriptionItem.spec.ts: Added a focused API test suite for the new Subscription Items module logic to ensure database actions and event emissions behave correctly for all 5 endpoints.

Type of Change

• Bug fix
• New feature
• Breaking change
• Refactoring
• Documentation

How Was This Tested?

• Unit tests
• Integration tests
• Manual testing

Tested all five CRUD operations via the new SubscriptionItem.spec.ts test suite. Ran npx nx test api to ensure all existing test suites pass. Manually verified route registration and authentication rejection (401 vs 404) via terminal cURL.

Screenshots

All test case passed

Terminal output showing the new /v1/subscription_items route returning 401 Unauthorized instead of 404 Not Found, proving the endpoint is correctly mounted.

Checklist

• Self-reviewed my code
• No new warnings
• Existing tests still pass
• Breaking changes are documented above

[Benzilla]

Thank you! the endpoints are looking good.

Few things I'd want before merging:

Missing items should 404, not 400 invalid_request_error. Stripe uses resource_missing here; we already do the same for invoice items (INVOICE_ITEM_NOT_FOUND). Can we add SUBSCRIPTION_ITEM_NOT_FOUND and use it on retrieve/update/delete?

Small things:

• import DeleteSubscriptionItemSchema at the top of Subscription.ts rather than the inline typeof import(...).
• Lets make the function names more explicit, e.g. CreateItem -> CreateSubscriptionItem, DeleteItem-> DeleteSubscriptionItem etc. to match naming conventions across other modules.

[Abhishek-singh88]

Thanks for the review and catching those, I just pushed a new commit addressing everything:

• 404 Errors: Added SUBSCRIPTION_ITEM_NOT_FOUND to our centralized errors so that missing items properly throw a 404 resource_missing on the retrieve, update, and delete endpoints to match Stripe's convention.
• Top-level Import: Moved DeleteSubscriptionItemSchema to the top-level imports in Subscription.ts.
• Explicit Naming: Renamed the public functions (CreateItem -> CreateSubscriptionItem, etc.) to align with the rest of the modules. (Note: Since there was already an internal helper named CreateSubscriptionItem, I renamed that one to InsertSubscriptionItem to safely avoid conflicts).

I've re run the tests locally and everything is perfectly green. Let me know if there's anything else you need before merging

[Benzilla]

Thanks so much! merging now.