fix(download): pin frontend link generation to fixed relay config

The runtime kept reading aksrt_download_config after the settings page was
removed, so the frontend still issued download links per the stale option
values instead of the built-in defaults.

- Pin endpoint, per-token uses and TTL to built-in constants and drop all
  option reads plus the is_enabled() master switch
- Remove the now-unreachable direct-download branches from the download
  box template and the paid-card download helper
- Lock the pinned contract in the download contract tests

templates/parts/resource-download-box.php

@@ -27,9 +27,6 @@ if ($pay_type !== 'download' || empty($downloads)) {
 $is_resource_context = function_exists('aksrt_is_resource_post') && aksrt_is_resource_post($post_id);
 $context_variant = $is_resource_context ? 'resource' : 'content';
 
-// 下载总开关关闭 = 直连模式：下载按钮直接输出原始地址，不走本站中转链接。
-$direct_download = class_exists('\AKSRT\Content\Download')
-	&& !\AKSRT\Content\Download::get_instance()->is_enabled();
 $subject_label = trim((string) ($meta['subject_label'] ?? ''));
 if ($subject_label === '') {
 	$subject_label = $is_resource_context ? __('资源', 'aksrt') : __('内容', 'aksrt');
@@ -60,9 +57,7 @@ $pay_modo = is_array($priced)
 $user_amount = is_array($priced) ? (float) ($priced['amount'] ?? 0) : (float) ($meta['price'] ?? 0);
 $cost = function_exists('aksrt_resource_download_cost') ? (int) aksrt_resource_download_cost($post_id) : 0;
 
-// 直连模式下没有“获取链接”请求，验证码容器不会被打开，属死 UI 不渲染。
-$captcha_active = !$direct_download
-	&& function_exists('aksrt_is_captcha_enabled_for')
+$captcha_active = function_exists('aksrt_is_captcha_enabled_for')
 	&& aksrt_is_captcha_enabled_for('download');
 $balance = (int) ($gate_data['balance'] ?? (is_user_logged_in() ? \AKSRT\Ledger\LedgerService::get_instance()->balance(get_current_user_id(), \AKSRT\Ledger\AssetType::POINTS) : 0));
 $login_url = (string) ($gate_data['login_url'] ?? wp_login_url(get_permalink($post_id)));
@@ -350,10 +345,6 @@ if ($is_need_purchase && $user_amount > 0) {
 					}
 					$type = isset($row['type']) ? (string) $row['type'] : 'direct';
 					$url = isset($row['url']) ? (string) $row['url'] : '';
-					if ($direct_download && $url === '') {
-						// 直连模式没有可用的原始地址，空源不渲染。
-						continue;
-					}
 					$extract_code = isset($row['extract_code']) ? trim((string) $row['extract_code']) : '';
 					$has_extract = $extract_code !== '';
 
@@ -371,15 +362,6 @@ if ($is_need_purchase && $user_amount > 0) {
 						$btn_action = '';
 						$btn_target = '';
 						$btn_rel = '';
-					} elseif ($direct_download) {
-						$btn_tag = 'a';
-						$btn_href = $url;
-						$btn_text = __('立即下载', 'aksrt');
-						$btn_icon = 'fa-download';
-						$btn_modifier = $has_paid_access ? ' resource-download-box__btn--owned' : '';
-						$btn_action = '';
-						$btn_target = ' target="_blank"';
-						$btn_rel = ' rel="nofollow noopener"';
 					} else {
 						$btn_tag = 'button';
 						$btn_href = '';
@@ -406,12 +388,6 @@ if ($is_need_purchase && $user_amount > 0) {
 									<?php echo esc_html($service['name']); ?>
 								</span>
 							</a>
-							<?php if ($direct_download && $has_extract) : ?>
-								<button type="button" class="resource-download-box__btn resource-download-box__btn--extract"
-									data-clipboard-tag="<?php esc_attr_e('提取码', 'aksrt'); ?>"
-									data-clipboard-text="<?php echo esc_attr($extract_code); ?>"
-								><?php echo esc_html($extract_code); ?></button>
-							<?php endif; ?>
 						<?php else : ?>
 							<button
 								type="button"