feat(admin): Add email search and rate limit filtering for accounts and redeem codes

- Add used_by_email column to redeem code export CSV for better user identification
- Implement rate_limited status filter in account listing with RateLimitResetAt check
- Extend redeem code search to include user email in addition to code matching
- Add API key search capability to user listing filters
- Display user email in redeem code table used_by column for improved visibility
- Update search placeholders in UI to reflect expanded search capabilities (email, username, notes, API key)
- Improve Chinese and English localization strings for search hints

backend/internal/repository/account_repo.go

@@ -448,7 +448,12 @@ func (r *accountRepository) ListWithFilters(ctx context.Context, params paginati
 		q = q.Where(dbaccount.TypeEQ(accountType))
 	}
 	if status != "" {
-		q = q.Where(dbaccount.StatusEQ(status))
+		switch status {
+		case "rate_limited":
+			q = q.Where(dbaccount.RateLimitResetAtGT(time.Now()))
+		default:
+			q = q.Where(dbaccount.StatusEQ(status))
+		}
 	}
 	if search != "" {
 		q = q.Where(dbaccount.NameContainsFold(search))