refactor(affiliate): tighten DI and harden inviter code validation

- Drop SetAffiliateService setters and ProvideAuthService /
  ProvidePaymentService / ProvideUserHandler wrappers in favor of direct
  Wire constructor injection. AffiliateService has no back-edge to
  Auth/Payment/User, so the indirection was never required.
- Change RegisterWithVerification's variadic affiliateCode to a fixed
  parameter; adjust all call sites.
- Validate aff_code length and charset in BindInviterByCode before any
  DB lookup, eliminating timing-side-channel and useless DB roundtrips
  on malformed input.
- Make affiliate cache invalidation synchronous; surface Redis errors
  via the project logger instead of swallowing them in a detached
  goroutine.
- Add an integration test guarding cross-layer tx propagation in
  AccrueQuota and a unit test pinning the aff_code format rules.

backend/internal/handler/wire.go

@@ -80,18 +80,6 @@ func ProvideSettingHandler(settingService *service.SettingService, buildInfo Bui
 	return NewSettingHandler(settingService, buildInfo.Version)
 }
 
-func ProvideUserHandler(
-	userService *service.UserService,
-	authService *service.AuthService,
-	emailService *service.EmailService,
-	emailCache service.EmailCache,
-	affiliateService *service.AffiliateService,
-) *UserHandler {
-	handler := NewUserHandler(userService, authService, emailService, emailCache)
-	handler.SetAffiliateService(affiliateService)
-	return handler
-}
-
 // ProvideHandlers creates the Handlers struct
 func ProvideHandlers(
 	authHandler *AuthHandler,
@@ -137,7 +125,7 @@ func ProvideHandlers(
 var ProviderSet = wire.NewSet(
 	// Top-level handlers
 	NewAuthHandler,
-	ProvideUserHandler,
+	NewUserHandler,
 	NewAPIKeyHandler,
 	NewUsageHandler,
 	NewRedeemHandler,