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/repository/api_key_repo.go

@@ -3,6 +3,7 @@ package repository
 import (
 	"context"
 	"database/sql"
+	"encoding/json"
 	"fmt"
 	"strings"
 	"time"
@@ -639,22 +640,32 @@ func userEntityToService(u *dbent.User) *service.User {
 	if u == nil {
 		return nil
 	}
-	return &service.User{
-		ID:                  u.ID,
-		Email:               u.Email,
-		Username:            u.Username,
-		Notes:               u.Notes,
-		PasswordHash:        u.PasswordHash,
-		Role:                u.Role,
-		Balance:             u.Balance,
-		Concurrency:         u.Concurrency,
-		Status:              u.Status,
-		TotpSecretEncrypted: u.TotpSecretEncrypted,
-		TotpEnabled:         u.TotpEnabled,
-		TotpEnabledAt:       u.TotpEnabledAt,
-		CreatedAt:           u.CreatedAt,
-		UpdatedAt:           u.UpdatedAt,
+	out := &service.User{
+		ID:                   u.ID,
+		Email:                u.Email,
+		Username:             u.Username,
+		Notes:                u.Notes,
+		PasswordHash:         u.PasswordHash,
+		Role:                 u.Role,
+		Balance:              u.Balance,
+		Concurrency:          u.Concurrency,
+		Status:               u.Status,
+		TotpSecretEncrypted:  u.TotpSecretEncrypted,
+		TotpEnabled:          u.TotpEnabled,
+		TotpEnabledAt:        u.TotpEnabledAt,
+		BalanceNotifyEnabled: u.BalanceNotifyEnabled,
+		BalanceNotifyThreshold: u.BalanceNotifyThreshold,
+		CreatedAt:            u.CreatedAt,
+		UpdatedAt:            u.UpdatedAt,
 	}
+	// Parse extra emails JSON array
+	if u.BalanceNotifyExtraEmails != "" && u.BalanceNotifyExtraEmails != "[]" {
+		var emails []string
+		if err := json.Unmarshal([]byte(u.BalanceNotifyExtraEmails), &emails); err == nil {
+			out.BalanceNotifyExtraEmails = emails
+		}
+	}
+	return out
 }
 
 func groupEntityToService(g *dbent.Group) *service.Group {

backend/internal/repository/email_cache.go

@@ -11,6 +11,7 @@ import (
 
 const (
 	verifyCodeKeyPrefix          = "verify_code:"
+	notifyVerifyKeyPrefix        = "notify_verify:"
 	passwordResetKeyPrefix       = "password_reset:"
 	passwordResetSentAtKeyPrefix = "password_reset_sent:"
 )
@@ -20,6 +21,11 @@ func verifyCodeKey(email string) string {
 	return verifyCodeKeyPrefix + email
 }
 
+// notifyVerifyKey generates the Redis key for notify email verification code.
+func notifyVerifyKey(email string) string {
+	return notifyVerifyKeyPrefix + email
+}
+
 // passwordResetKey generates the Redis key for password reset token.
 func passwordResetKey(email string) string {
 	return passwordResetKeyPrefix + email
@@ -106,3 +112,32 @@ func (c *emailCache) SetPasswordResetEmailCooldown(ctx context.Context, email st
 	key := passwordResetSentAtKey(email)
 	return c.rdb.Set(ctx, key, "1", ttl).Err()
 }
+
+// Notify email verification code methods
+
+func (c *emailCache) GetNotifyVerifyCode(ctx context.Context, email string) (*service.VerificationCodeData, error) {
+	key := notifyVerifyKey(email)
+	val, err := c.rdb.Get(ctx, key).Result()
+	if err != nil {
+		return nil, err
+	}
+	var data service.VerificationCodeData
+	if err := json.Unmarshal([]byte(val), &data); err != nil {
+		return nil, err
+	}
+	return &data, nil
+}
+
+func (c *emailCache) SetNotifyVerifyCode(ctx context.Context, email string, data *service.VerificationCodeData, ttl time.Duration) error {
+	key := notifyVerifyKey(email)
+	val, err := json.Marshal(data)
+	if err != nil {
+		return err
+	}
+	return c.rdb.Set(ctx, key, val, ttl).Err()
+}
+
+func (c *emailCache) DeleteNotifyVerifyCode(ctx context.Context, email string) error {
+	key := notifyVerifyKey(email)
+	return c.rdb.Del(ctx, key).Err()
+}

backend/internal/repository/user_repo.go

@@ -3,6 +3,7 @@ package repository
 import (
 	"context"
 	"database/sql"
+	"encoding/json"
 	"errors"
 	"fmt"
 	"sort"
@@ -137,7 +138,7 @@ func (r *userRepository) Update(ctx context.Context, userIn *service.User) error
 		txClient = r.client
 	}
 
-	updated, err := txClient.User.UpdateOneID(userIn.ID).
+	updateOp := txClient.User.UpdateOneID(userIn.ID).
 		SetEmail(userIn.Email).
 		SetUsername(userIn.Username).
 		SetNotes(userIn.Notes).
@@ -146,7 +147,13 @@ func (r *userRepository) Update(ctx context.Context, userIn *service.User) error
 		SetBalance(userIn.Balance).
 		SetConcurrency(userIn.Concurrency).
 		SetStatus(userIn.Status).
-		Save(ctx)
+		SetBalanceNotifyEnabled(userIn.BalanceNotifyEnabled).
+		SetNillableBalanceNotifyThreshold(userIn.BalanceNotifyThreshold).
+		SetBalanceNotifyExtraEmails(marshalExtraEmails(userIn.BalanceNotifyExtraEmails))
+	if userIn.BalanceNotifyThreshold == nil {
+		updateOp = updateOp.ClearBalanceNotifyThreshold()
+	}
+	updated, err := updateOp.Save(ctx)
 	if err != nil {
 		return translatePersistenceError(err, service.ErrUserNotFound, service.ErrEmailExists)
 	}
@@ -549,6 +556,18 @@ func applyUserEntityToService(dst *service.User, src *dbent.User) {
 	dst.UpdatedAt = src.UpdatedAt
 }
 
+// marshalExtraEmails serializes a string slice to JSON for storage.
+func marshalExtraEmails(emails []string) string {
+	if len(emails) == 0 {
+		return "[]"
+	}
+	data, err := json.Marshal(emails)
+	if err != nil {
+		return "[]"
+	}
+	return string(data)
+}
+
 // UpdateTotpSecret 更新用户的 TOTP 加密密钥
 func (r *userRepository) UpdateTotpSecret(ctx context.Context, userID int64, encryptedSecret *string) error {
 	client := clientFromContext(ctx, r.client)