feat: add configurable watermark text in API settings

app/templates/api_settings.html

@@ -57,6 +57,12 @@
                 <small class="text-muted">控制输出的随机性，值越大越有创造性</small>
             </div>
 
+            <div class="mb-3">
+                <label class="form-label">PDF水印文本</label>
+                <input type="text" class="form-control" id="watermarkText" placeholder="留空则不显示水印">
+                <small class="text-muted">PDF每页中央显示的斜向水印文字</small>
+            </div>
+
             <div class="d-flex gap-2">
                 <button type="button" class="btn btn-primary" onclick="saveApiConfig()">
                     <i class="bi bi-save"></i> 保存配置
@@ -171,7 +177,8 @@ async function saveApiConfig(silent = false, overrideProvider = null) {
         model: document.getElementById('apiModel').value,
         api_key: document.getElementById('apiKey').value,
         base_url: document.getElementById('apiEndpoint').value,
-        temperature: parseFloat(document.getElementById('apiTemperature').value)
+        temperature: parseFloat(document.getElementById('apiTemperature').value),
+        watermark_text: document.getElementById('watermarkText').value.trim()
     };
 
     const r = await fetch('/api/config', {
@@ -199,6 +206,7 @@ async function loadApiConfig() {
         document.getElementById('apiKey').value = config.api_key || '';
         document.getElementById('apiEndpoint').value = config.base_url || '';
         document.getElementById('apiTemperature').value = config.temperature || 0.7;
+        document.getElementById('watermarkText').value = config.watermark_text || '';
 
         if (config.api_key_preview) {
             document.getElementById('apiKeyPreview').textContent = '当前: ' + config.api_key_preview;