feat(notify): add balance low & account quota notification system

- User balance low notification: email alert when balance drops below
  configurable threshold (user email + verified extra emails)
- Account quota notification: broadcast email to admin-configured
  recipients when daily/weekly/total quota usage exceeds alert threshold
- Admin settings: global enable/disable, default threshold, quota
  notification email list (Email Settings tab)
- User profile: enable/disable, custom threshold, add/remove extra
  notification emails with verification code flow
- Account quota: per-dimension alert toggle and threshold in quota
  control card
- Trigger logic: first-crossing only (old >= threshold && new < threshold
  for balance; old < threshold && new >= threshold for quota), naturally
  prevents duplicate notifications without Redis dedup

backend/internal/service/user_service_test.go

@@ -114,7 +114,7 @@ func (m *mockBillingCache) InvalidateAPIKeyRateLimit(context.Context, int64) err
 func TestUpdateBalance_Success(t *testing.T) {
 	repo := &mockUserRepo{}
 	cache := &mockBillingCache{}
-	svc := NewUserService(repo, nil, cache)
+	svc := NewUserService(repo, nil, nil, cache)
 
 	err := svc.UpdateBalance(context.Background(), 42, 100.0)
 	require.NoError(t, err)
@@ -131,7 +131,7 @@ func TestUpdateBalance_Success(t *testing.T) {
 
 func TestUpdateBalance_NilBillingCache_NoPanic(t *testing.T) {
 	repo := &mockUserRepo{}
-	svc := NewUserService(repo, nil, nil) // billingCache = nil
+	svc := NewUserService(repo, nil, nil, nil) // billingCache = nil
 
 	err := svc.UpdateBalance(context.Background(), 1, 50.0)
 	require.NoError(t, err, "billingCache 为 nil 时不应 panic")
@@ -140,7 +140,7 @@ func TestUpdateBalance_NilBillingCache_NoPanic(t *testing.T) {
 func TestUpdateBalance_CacheFailure_DoesNotAffectReturn(t *testing.T) {
 	repo := &mockUserRepo{}
 	cache := &mockBillingCache{invalidateErr: errors.New("redis connection refused")}
-	svc := NewUserService(repo, nil, cache)
+	svc := NewUserService(repo, nil, nil, cache)
 
 	err := svc.UpdateBalance(context.Background(), 99, 200.0)
 	require.NoError(t, err, "缓存失效失败不应影响主流程返回值")
@@ -154,7 +154,7 @@ func TestUpdateBalance_CacheFailure_DoesNotAffectReturn(t *testing.T) {
 func TestUpdateBalance_RepoError_ReturnsError(t *testing.T) {
 	repo := &mockUserRepo{updateBalanceErr: errors.New("database error")}
 	cache := &mockBillingCache{}
-	svc := NewUserService(repo, nil, cache)
+	svc := NewUserService(repo, nil, nil, cache)
 
 	err := svc.UpdateBalance(context.Background(), 1, 100.0)
 	require.Error(t, err, "repo 失败时应返回错误")
@@ -170,7 +170,7 @@ func TestUpdateBalance_WithAuthCacheInvalidator(t *testing.T) {
 	repo := &mockUserRepo{}
 	auth := &mockAuthCacheInvalidator{}
 	cache := &mockBillingCache{}
-	svc := NewUserService(repo, auth, cache)
+	svc := NewUserService(repo, nil, auth, cache)
 
 	err := svc.UpdateBalance(context.Background(), 77, 300.0)
 	require.NoError(t, err)
@@ -191,7 +191,7 @@ func TestNewUserService_FieldsAssignment(t *testing.T) {
 	auth := &mockAuthCacheInvalidator{}
 	cache := &mockBillingCache{}
 
-	svc := NewUserService(repo, auth, cache)
+	svc := NewUserService(repo, nil, auth, cache)
 	require.NotNil(t, svc)
 	require.Equal(t, repo, svc.userRepo)
 	require.Equal(t, auth, svc.authCacheInvalidator)