feat(server): API relay gateway backend M0-M4

Gin + GORM + pure-Go SQLite. Users/auth (JWT), API key management with
quotas, proxy gateway with weighted channel failover and health checks,
usage/billing ledger, cross-protocol conversion (Anthropic Messages /
OpenAI Chat Completions / OpenAI Responses), and channel/model admin API.
Channels declare native API formats and auto-convert the rest.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

server/internal/billing/service.go

@@ -0,0 +1,117 @@
+package billing
+
+import (
+	"errors"
+	"time"
+
+	"github.com/shopspring/decimal"
+	"go.uber.org/zap"
+	"gorm.io/gorm"
+	"gorm.io/gorm/clause"
+
+	"openteam/server/internal/store"
+)
+
+var (
+	ErrInsufficientBalance = errors.New("insufficient balance")
+	ErrUserNotFound        = errors.New("user not found")
+)
+
+// PriceSnapshot mirrors a model's prices at billing time.
+type PriceSnapshot struct {
+	InputPrice     decimal.Decimal
+	OutputPrice    decimal.Decimal
+	CacheReadPrice decimal.Decimal
+}
+
+// CostFromPrices computes cost for token counts using per-1M-token prices.
+func CostFromPrices(in, out, cacheRead int64, prices PriceSnapshot) decimal.Decimal {
+	perM := decimal.NewFromInt(1_000_000)
+	cost := prices.InputPrice.Mul(decimal.NewFromInt(in)).Div(perM).
+		Add(prices.OutputPrice.Mul(decimal.NewFromInt(out)).Div(perM)).
+		Add(prices.CacheReadPrice.Mul(decimal.NewFromInt(cacheRead)).Div(perM))
+	return cost.Round(8)
+}
+
+// EstimateCost approximates cost from token estimates.
+func (s *Service) EstimateCost(modelID int64, in, out, cacheRead int64) (decimal.Decimal, error) {
+	var m store.Model
+	if err := s.db.First(&m, modelID).Error; err != nil {
+		return decimal.Zero, err
+	}
+	return CostFromPrices(in, out, cacheRead, PriceSnapshot{
+		InputPrice: m.InputPrice, OutputPrice: m.OutputPrice, CacheReadPrice: m.CacheReadPrice,
+	}), nil
+}
+
+type Service struct {
+	db  *gorm.DB
+	log *zap.Logger
+}
+
+func NewService(db *gorm.DB, log *zap.Logger) *Service {
+	return &Service{db: db, log: log}
+}
+
+// CheckBalance returns whether the user can afford the estimated cost.
+func (s *Service) CheckBalance(userID int64, estimated decimal.Decimal) error {
+	var u store.User
+	if err := s.db.First(&u, userID).Error; err != nil {
+		if errors.Is(err, gorm.ErrRecordNotFound) {
+			return ErrUserNotFound
+		}
+		return err
+	}
+	if u.Balance.LessThan(estimated) {
+		return ErrInsufficientBalance
+	}
+	return nil
+}
+
+// Deduct atomically debits the user balance and appends a ledger entry.
+func (s *Service) Deduct(userID int64, change decimal.Decimal, logType, refID string) (decimal.Decimal, error) {
+	return s.mutateBalance(userID, change.Neg(), logType, refID)
+}
+
+// Credit adds to the user balance (recharge / refund / admin adjust).
+func (s *Service) Credit(userID int64, change decimal.Decimal, logType, refID string) (decimal.Decimal, error) {
+	return s.mutateBalance(userID, change, logType, refID)
+}
+
+// mutateBalance performs the balance update in a transaction so ledger and
+// balance always agree.
+func (s *Service) mutateBalance(userID int64, delta decimal.Decimal, logType, refID string) (decimal.Decimal, error) {
+	var after decimal.Decimal
+	err := s.db.Transaction(func(tx *gorm.DB) error {
+		var u store.User
+		if err := tx.Clauses(clause.Locking{Strength: "UPDATE"}).First(&u, userID).Error; err != nil {
+			return err
+		}
+		newBal := u.Balance.Add(delta)
+		if newBal.LessThan(decimal.Zero) {
+			return ErrInsufficientBalance
+		}
+		if err := tx.Model(&u).Update("balance", newBal).Error; err != nil {
+			return err
+		}
+		after = newBal
+		return tx.Create(&store.BalanceLog{
+			UserID:       userID,
+			Change:       delta,
+			BalanceAfter: newBal,
+			Type:         logType,
+			RefID:        refID,
+			CreatedAt:    time.Now(),
+		}).Error
+	})
+	if err != nil {
+		return decimal.Zero, err
+	}
+	return after, nil
+}
+
+// AdminAdjust changes a user's balance with an optional reason.
+func (s *Service) AdminAdjust(userID int64, amount decimal.Decimal, reason string) (decimal.Decimal, error) {
+	return s.mutateBalance(userID, amount, "admin_adjust", reason)
+}
+