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

app/api/v1/images.py

@@ -30,6 +30,19 @@ from app.services.llm_service import LLMService
 logger = logging.getLogger(__name__)
 router = APIRouter()
 
+# OCR 并发控制信号量（动态读取配置）
+_ocr_semaphore: asyncio.Semaphore | None = None
+
+
+def _get_ocr_semaphore() -> asyncio.Semaphore:
+    """获取/重建 OCR 并发信号量"""
+    global _ocr_semaphore
+    from app.config import settings
+    concurrency = settings.OCR_CONCURRENCY
+    if _ocr_semaphore is None or _ocr_semaphore._value != concurrency:
+        _ocr_semaphore = asyncio.Semaphore(concurrency)
+    return _ocr_semaphore
+
 ALLOWED_EXTENSIONS = {".png", ".jpg", ".jpeg", ".bmp", ".webp"}
 
 # 全局 BM25 索引实例（在 main.py 中初始化）
@@ -126,7 +139,8 @@ async def recognize_image(image_id: int, db: AsyncSession = Depends(get_db)):
     await db.flush()
 
     try:
-        ocr_result = await OCRService.recognize(image_record.file_path)
+        async with _get_ocr_semaphore():
+            ocr_result = await OCRService.recognize(image_record.file_path)
         _save_ocr_result(image_record, ocr_result, db)
         await db.flush()
         await db.refresh(image_record)
@@ -171,7 +185,8 @@ async def batch_recognize(files: List[UploadFile], db: AsyncSession = Depends(ge
         db.add(image_record)
         await db.flush()
         try:
-            ocr_result = await OCRService.recognize(str(dest_path))
+            async with _get_ocr_semaphore():
+                ocr_result = await OCRService.recognize(str(dest_path))
             # OCR 内容去重
             existing_id = await _check_ocr_duplicate(ocr_result.text, db)
             if existing_id is not None:
@@ -239,7 +254,8 @@ async def import_from_paths(data: PathImportRequest, db: AsyncSession = Depends(
         db.add(image_record)
         await db.flush()
         try:
-            ocr_result = await OCRService.recognize(file_path)
+            async with _get_ocr_semaphore():
+                ocr_result = await OCRService.recognize(file_path)
             # OCR 内容去重
             existing_id = await _check_ocr_duplicate(ocr_result.text, db)
             if existing_id is not None:
@@ -279,7 +295,8 @@ async def recognize_direct(file: UploadFile, db: AsyncSession = Depends(get_db))
     db.add(image_record)
     await db.flush()
     try:
-        ocr_result = await OCRService.recognize(str(dest_path))
+        async with _get_ocr_semaphore():
+            ocr_result = await OCRService.recognize(str(dest_path))
         # OCR 内容去重
         existing_id = await _check_ocr_duplicate(ocr_result.text, db)
         if existing_id is not None: