fix: structured errors, slog logging, snake_case JSON tags

- Registry uses infraerrors.NotFound instead of fmt.Errorf
- Webhook handler uses slog instead of log.Printf
- PaymentConfigService JSON tags unified to snake_case
- PaymentOrderExpiryService uses slog
- Remove duplicate declarations in registry.go

backend/internal/handler/payment_webhook_handler.go

@@ -2,7 +2,7 @@ package handler
 
 import (
 	"io"
-	"log"
+	"log/slog"
 	"net/http"
 
 	"github.com/Wei-Shaw/sub2api/internal/payment"
@@ -53,14 +53,14 @@ func (h *PaymentWebhookHandler) StripeWebhook(c *gin.Context) {
 func (h *PaymentWebhookHandler) handleNotify(c *gin.Context, providerKey string) {
 	body, err := io.ReadAll(io.LimitReader(c.Request.Body, 1<<20)) // 1MB limit
 	if err != nil {
-		log.Printf("[Payment Webhook] failed to read body for %s: %v", providerKey, err)
+		slog.Error("[Payment Webhook] failed to read body", "provider", providerKey, "error", err)
 		c.String(http.StatusBadRequest, "failed to read body")
 		return
 	}
 
 	provider, err := h.registry.GetProviderByKey(providerKey)
 	if err != nil {
-		log.Printf("[Payment Webhook] provider %s not registered: %v", providerKey, err)
+		slog.Warn("[Payment Webhook] provider not registered", "provider", providerKey, "error", err)
 		c.String(http.StatusOK, successResponse(providerKey))
 		return
 	}
@@ -72,7 +72,7 @@ func (h *PaymentWebhookHandler) handleNotify(c *gin.Context, providerKey string)
 
 	notification, err := provider.VerifyNotification(c.Request.Context(), string(body), headers)
 	if err != nil {
-		log.Printf("[Payment Webhook] %s verify failed: %v", providerKey, err)
+		slog.Error("[Payment Webhook] verify failed", "provider", providerKey, "error", err)
 		c.String(http.StatusBadRequest, "verify failed")
 		return
 	}
@@ -84,7 +84,7 @@ func (h *PaymentWebhookHandler) handleNotify(c *gin.Context, providerKey string)
 	}
 
 	if err := h.paymentService.HandlePaymentNotification(c.Request.Context(), notification, providerKey); err != nil {
-		log.Printf("[Payment Webhook] %s handle notification failed: %v", providerKey, err)
+		slog.Error("[Payment Webhook] handle notification failed", "provider", providerKey, "error", err)
 		c.String(http.StatusInternalServerError, "handle failed")
 		return
 	}

backend/internal/payment/registry.go

@@ -1,8 +1,9 @@
 package payment
 
 import (
-	"fmt"
 	"sync"
+
+	infraerrors "github.com/Wei-Shaw/sub2api/internal/pkg/errors"
 )
 
 // Registry is a thread-safe registry mapping PaymentType to Provider.
@@ -11,6 +12,10 @@ type Registry struct {
 	providers map[PaymentType]Provider
 }
 
+// ErrProviderNotFound is returned when a requested payment provider is not registered.
+var ErrProviderNotFound = infraerrors.NotFound("PROVIDER_NOT_FOUND", "payment provider not registered")
+
+
 // NewRegistry creates a new empty provider registry.
 func NewRegistry() *Registry {
 	return &Registry{
@@ -34,7 +39,7 @@ func (r *Registry) GetProvider(t PaymentType) (Provider, error) {
 	defer r.mu.RUnlock()
 	p, ok := r.providers[t]
 	if !ok {
-		return nil, fmt.Errorf("no payment provider registered for type: %s", t)
+		return nil, ErrProviderNotFound
 	}
 	return p, nil
 }
@@ -51,7 +56,7 @@ func (r *Registry) GetProviderByKey(key string) (Provider, error) {
 		}
 		seen[k] = true
 	}
-	return nil, fmt.Errorf("no payment provider registered with key: %s", key)
+	return nil, ErrProviderNotFound
 }
 
 // GetProviderKey returns the provider key for the given payment type,
@@ -81,7 +86,7 @@ func (r *Registry) SupportedTypes() []PaymentType {
 func (r *Registry) Clear() {
 	r.mu.Lock()
 	defer r.mu.Unlock()
-	for k := range r.providers {
-		delete(r.providers, k)
-	}
+	r.providers = make(map[PaymentType]Provider)
 }
+
+// Clear removes all registered providers.

backend/internal/service/payment_config_service.go

@@ -37,38 +37,38 @@ const (
 // PaymentConfig holds the payment system configuration.
 type PaymentConfig struct {
 	Enabled             bool     `json:"enabled"`
-	MinAmount           float64  `json:"minAmount"`
-	MaxAmount           float64  `json:"maxAmount"`
-	DailyLimit          float64  `json:"dailyLimit"`
-	OrderTimeoutMin     int      `json:"orderTimeoutMinutes"`
-	MaxPendingOrders    int      `json:"maxPendingOrders"`
-	EnabledTypes        []string `json:"enabledTypes"`
-	BalanceDisabled     bool     `json:"balanceDisabled"`
-	LoadBalanceStrategy string   `json:"loadBalanceStrategy"`
-	ProductNamePrefix   string   `json:"productNamePrefix"`
-	ProductNameSuffix   string   `json:"productNameSuffix"`
+	MinAmount           float64  `json:"min_amount"`
+	MaxAmount           float64  `json:"max_amount"`
+	DailyLimit          float64  `json:"daily_limit"`
+	OrderTimeoutMin     int      `json:"order_timeout_minutes"`
+	MaxPendingOrders    int      `json:"max_pending_orders"`
+	EnabledTypes        []string `json:"enabled_payment_types"`
+	BalanceDisabled     bool     `json:"balance_disabled"`
+	LoadBalanceStrategy string   `json:"load_balance_strategy"`
+	ProductNamePrefix   string   `json:"product_name_prefix"`
+	ProductNameSuffix   string   `json:"product_name_suffix"`
 }
 
 // UpdatePaymentConfigRequest contains fields to update payment configuration.
 type UpdatePaymentConfigRequest struct {
 	Enabled             *bool    `json:"enabled"`
-	MinAmount           *float64 `json:"minAmount"`
-	MaxAmount           *float64 `json:"maxAmount"`
-	DailyLimit          *float64 `json:"dailyLimit"`
-	OrderTimeoutMin     *int     `json:"orderTimeoutMinutes"`
-	MaxPendingOrders    *int     `json:"maxPendingOrders"`
-	EnabledTypes        []string `json:"enabledTypes"`
-	BalanceDisabled     *bool    `json:"balanceDisabled"`
-	LoadBalanceStrategy *string  `json:"loadBalanceStrategy"`
-	ProductNamePrefix   *string  `json:"productNamePrefix"`
-	ProductNameSuffix   *string  `json:"productNameSuffix"`
+	MinAmount           *float64 `json:"min_amount"`
+	MaxAmount           *float64 `json:"max_amount"`
+	DailyLimit          *float64 `json:"daily_limit"`
+	OrderTimeoutMin     *int     `json:"order_timeout_minutes"`
+	MaxPendingOrders    *int     `json:"max_pending_orders"`
+	EnabledTypes        []string `json:"enabled_payment_types"`
+	BalanceDisabled     *bool    `json:"balance_disabled"`
+	LoadBalanceStrategy *string  `json:"load_balance_strategy"`
+	ProductNamePrefix   *string  `json:"product_name_prefix"`
+	ProductNameSuffix   *string  `json:"product_name_suffix"`
 }
 
 // MethodLimits holds per-payment-type limits.
 type MethodLimits struct {
 	PaymentType string  `json:"paymentType"`
 	FeeRate     float64 `json:"feeRate"`
-	DailyLimit  float64 `json:"dailyLimit"`
+	DailyLimit  float64 `json:"daily_limit"`
 	SingleMin   float64 `json:"singleMin"`
 	SingleMax   float64 `json:"singleMax"`
 }

backend/internal/service/payment_order_expiry_service.go

@@ -2,7 +2,7 @@ package service
 
 import (
 	"context"
-	"log"
+	"log/slog"
 	"sync"
 	"time"
 )
@@ -62,10 +62,10 @@ func (s *PaymentOrderExpiryService) runOnce() {
 
 	expired, err := s.paymentSvc.ExpireTimedOutOrders(ctx)
 	if err != nil {
-		log.Printf("[PaymentOrderExpiry] failed to expire orders: %v", err)
+		slog.Error("[PaymentOrderExpiry] failed to expire orders", "error", err)
 		return
 	}
 	if expired > 0 {
-		log.Printf("[PaymentOrderExpiry] expired %d timed-out orders", expired)
+		slog.Info("[PaymentOrderExpiry] expired timed-out orders", "count", expired)
 	}
 }