refactor(workspace): enforce layered crate boundaries

crates/aether-data/contracts/src/repository/billing/mod.rs

@@ -0,0 +1,9 @@
+mod types;
+
+pub use types::{
+    AdminBillingCollectorRecord, AdminBillingCollectorWriteInput, AdminBillingMutationOutcome,
+    AdminBillingPresetApplyResult, AdminBillingRuleRecord, AdminBillingRuleWriteInput,
+    BillingPlanRecord, BillingPlanWriteInput, BillingReadRepository, PaymentGatewayConfigRecord,
+    PaymentGatewayConfigWriteInput, StoredBillingModelContext, UserDailyQuotaAvailabilityRecord,
+    UserPlanEntitlementRecord,
+};