feat: OCR 并发识别控制，支持多用户同时上传

- config.py: 新增 OCR_CONCURRENCY 参数（默认1，范围1-10）
- images.py: 用 asyncio.Semaphore 控制 OCR 并发，4个上传端点统一受控
- settings API: 新增 ocr_concurrency 字段读写
- 前端设置页: 新增 OCR 并发数量输入框
- .env / .env.example: 新增 OCR_CONCURRENCY 配置项
- Semaphore 动态重建：修改配置后立即生效，无需重启

static/index.html

@@ -1093,6 +1093,14 @@
                                 <span style="color:var(--text-secondary); font-size:13px;">篇（1-50）</span>
                             </div>
                         </div>
+                        <div class="form-group">
+                            <label class="form-label">OCR 并发识别数量</label>
+                            <div style="display:flex; align-items:center; gap:8px;">
+                                <input type="number" id="setting-ocr-concurrency" min="1" max="10" value="1"
+                                       style="width:80px;" class="input-field">
+                                <span style="color:var(--text-secondary); font-size:13px;">张（1-10，多用户同时上传时控制并发）</span>
+                            </div>
+                        </div>
                     </div>
                 </div>
 
@@ -2644,6 +2652,9 @@
             if (data.judge_batch_size != null) {
                 document.getElementById('setting-judge-batch-size').value = data.judge_batch_size;
             }
+            if (data.ocr_concurrency != null) {
+                document.getElementById('setting-ocr-concurrency').value = data.ocr_concurrency;
+            }
             // 企业微信机器人
             document.getElementById('setting-wework-bot-enabled').checked = data.wework_bot_enabled;
             document.getElementById('setting-wework-bot-id').value = data.wework_bot_id || '';
@@ -2673,6 +2684,10 @@
         if (!isNaN(judgeBatchSize) && judgeBatchSize >= 1 && judgeBatchSize <= 50) {
             body.judge_batch_size = judgeBatchSize;
         }
+        const ocrConcurrency = parseInt(document.getElementById('setting-ocr-concurrency').value);
+        if (!isNaN(ocrConcurrency) && ocrConcurrency >= 1 && ocrConcurrency <= 10) {
+            body.ocr_concurrency = ocrConcurrency;
+        }
         // 企业微信机器人
         body.wework_bot_enabled = document.getElementById('setting-wework-bot-enabled').checked;
         const botId = document.getElementById('setting-wework-bot-id').value.trim();