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/settings.py

@@ -38,6 +38,7 @@ class SettingsResponse(BaseModel):
     chunk_overlap: int
     search_limit: int
     judge_batch_size: int
+    ocr_concurrency: int
     # API Key 只返回是否已配置（不返回实际值）
     has_minimax_key: bool
     has_deepseek_key: bool
@@ -71,6 +72,7 @@ async def get_settings():
         chunk_overlap=settings.CHUNK_OVERLAP,
         search_limit=settings.SEARCH_LIMIT,
         judge_batch_size=settings.JUDGE_BATCH_SIZE,
+        ocr_concurrency=settings.OCR_CONCURRENCY,
         has_minimax_key=bool(settings.MINIMAX_API_KEY),
         has_deepseek_key=bool(settings.DEEPSEEK_API_KEY),
         has_openai_key=bool(settings.OPENAI_API_KEY),
@@ -110,6 +112,7 @@ class SettingsUpdate(BaseModel):
     chunk_overlap: int | None = None
     search_limit: int | None = None
     judge_batch_size: int | None = None
+    ocr_concurrency: int | None = None
     # 企业微信机器人
     wework_bot_enabled: bool | None = None
     wework_bot_id: str | None = None
@@ -159,6 +162,7 @@ async def update_settings(data: SettingsUpdate):
         "chunk_overlap": "CHUNK_OVERLAP",
         "search_limit": "SEARCH_LIMIT",
         "judge_batch_size": "JUDGE_BATCH_SIZE",
+        "ocr_concurrency": "OCR_CONCURRENCY",
         "wework_bot_enabled": "WEWORK_BOT_ENABLED",
         "wework_bot_id": "WEWORK_BOT_ID",
         "wework_bot_secret": "WEWORK_BOT_SECRET",