make bulk coupon relations and naming easier to understand

[PNJS-35](https://linear.app/skillrecordings/issue/PNJS-35/redeemedbulkcouponid-empty-after-claiming-a-team-seat) we had an extra column on coupon for legacy reasons called `bulkPurchaseId` that previously linked a bulk coupon (used for redeeming team seats) with a **single** purchase. We've since added the idea that a single coupon spans multiple purchases for bulk and purchasing more adds to the maxUses of the single coupon. the coupon has a relation to `bulkCouponPurchases` and `bulkPurchases` which is also confusing so `bulkCouponPurchases` will change to `redeemedBulkCouponPurchases` to more robustly describe what's the actual relationship and make it easier to distinguish.