feat: complete remaining Go backend modules

backend/internal/billing/commit.go

@@ -0,0 +1,8 @@
+package billing
+
+import "errors"
+
+// ErrCommitOutcomeUnknown means a database commit response was lost and the
+// durable billing outcome could not be reconciled. Callers must not write a
+// compensating failure state because the charge may already have committed.
+var ErrCommitOutcomeUnknown = errors.New("billing commit outcome unknown")