Update Subscription Items

Update subscription items with proration handling. Handles mid-cycle subscription changes (upgrades/downgrades) with three proration behaviors: - **always_invoice**: Creates invoice immediately and attempts payment now. Use when you want the customer to pay for the upgrade immediately. - **create_prorations**: Creates floating items (invoice_id=NULL) that will be collected at the next renewal. Use for deferred billing. - **none**: No proration, changes apply at next renewal only. Items are updated but no proration charges/credits are created. Supports six item operations: - **Add new item**: `{"price_id": "price_xxx", "quantity": 2}` - **Add new item at period end**: `{"price_id": "price_xxx", "quantity": 2, "start_at_end": true}` - Created inactive, activated at next renewal - **Update existing item**: `{"id": "si_xxx", "quantity": 5}` or `{"id": "si_xxx", "price_id": "price_yyy"}` - **Delete item now**: `{"id": "si_xxx", "deleted": true}` - **Remove at period end**: `{"id": "si_xxx", "drop_at_end": true}` - Item removed at next renewal - **Cancel scheduled removal**: `{"id": "si_xxx", "drop_at_end": false}` - Cancel pending removal The endpoint calculates proration based on remaining time in the current billing period: - Credits are created for unused time on old/deleted items (negative amounts) - Charges are created for remaining time on new/updated items (positive amounts) Works for both User and API Key authentication. Account access is automatically validated via endpoint dependency.