refactor(docs): move integration doc to docs/ and add download link in settings

- Move ADMIN_PAYMENT_INTEGRATION_API.md → docs/ADMIN_PAYMENT_INTEGRATION_API.md
- Update README.md reference path
- Add payment integration doc download link in admin settings UI (Purchase section)
- Add i18n keys: integrationDoc / integrationDocHint (zh + en)

frontend/src/views/admin/SettingsView.vue

@@ -991,6 +991,26 @@
                 {{ t('admin.settings.purchase.iframeWarning') }}
               </p>
             </div>
+
+            <!-- Integration Docs -->
+            <div class="flex items-center gap-2 text-sm">
+              <svg xmlns="http://www.w3.org/2000/svg" class="h-4 w-4 shrink-0 text-gray-400" fill="none" viewBox="0 0 24 24" stroke="currentColor">
+                <path stroke-linecap="round" stroke-linejoin="round" stroke-width="2" d="M9 12h6m-6 4h6m2 5H7a2 2 0 01-2-2V5a2 2 0 012-2h5.586a1 1 0 01.707.293l5.414 5.414a1 1 0 01.293.707V19a2 2 0 01-2 2z" />
+              </svg>
+              <a
+                href="https://raw.githubusercontent.com/Wei-Shaw/sub2api/main/docs/ADMIN_PAYMENT_INTEGRATION_API.md"
+                target="_blank"
+                rel="noopener noreferrer"
+                class="text-blue-600 hover:underline dark:text-blue-400"
+                download="ADMIN_PAYMENT_INTEGRATION_API.md"
+              >
+                {{ t('admin.settings.purchase.integrationDoc') }}
+              </a>
+              <span class="text-gray-400 dark:text-gray-500">—</span>
+              <span class="text-xs text-gray-500 dark:text-gray-400">
+                {{ t('admin.settings.purchase.integrationDocHint') }}
+              </span>
+            </div>
           </div>
         </div>