refactor: serve static frontend with Go APIs

backend/internal/billing/http_service.go

@@ -92,8 +92,8 @@ type AdjustmentCommand struct {
 	AmountFen, DeltaFen                         int64
 }
 type AdjustmentResult struct {
-	Wallet Wallet
-	Entry  LedgerEntry
+	Wallet Wallet      `json:"wallet"`
+	Entry  LedgerEntry `json:"entry"`
 }
 
 type AccountConfigStore interface {