feat: Improve decimal precision for quota and payment calculations

- Added github.com/shopspring/decimal for precise floating-point calculations
- Refactored quota and payment calculations in multiple files to use decimal arithmetic
- Updated go.mod and go.sum to include decimal library
- Improved precision in topup, relay, and quota service calculations
- Added support for more OpenAI model variants in cache ratio settings

go.mod

@@ -22,12 +22,12 @@ require (
 	github.com/golang-jwt/jwt v3.2.2+incompatible
 	github.com/google/uuid v1.6.0
 	github.com/gorilla/websocket v1.5.0
-	github.com/jinzhu/copier v0.4.0
 	github.com/joho/godotenv v1.5.1
 	github.com/pkg/errors v0.9.1
 	github.com/pkoukk/tiktoken-go v0.1.7
 	github.com/samber/lo v1.39.0
 	github.com/shirou/gopsutil v3.21.11+incompatible
+	github.com/shopspring/decimal v1.4.0
 	golang.org/x/crypto v0.27.0
 	golang.org/x/image v0.23.0
 	golang.org/x/net v0.28.0