feat(server): 系统缓存清理接口与 OCR 文本层兜底增强

- 新增 system_cache 模块与 POST /settings/cache/clear，管理员可一键清理 OCR 结果/运行时配置/模型失败冷却/知识库索引/地点语义等进程内缓存
- 各服务暴露 clear_*_cache 方法（ocr/runtime_settings/runtime_chat/knowledge/application_location_semantic），SettingsCacheClearRead 汇总清理项
- OCR 转图片失败时尝试用 PDF 文本层兜底构建识别文档（有效字符≥8），并写结果缓存；OcrService 暴露 clear_result_cache
- receipt_folder 车票过滤补充身份证号关键词，附件文档/操作/展示模块同步适配
- 新增 system_cache_endpoints 测试，更新 openapi_schema/ocr/receipt_folder/attachment_association_jobs 测试

server/src/app/api/v1/endpoints/settings.py

@@ -5,18 +5,20 @@ from typing import Annotated
 from fastapi import APIRouter, Depends, Header, HTTPException, status
 from sqlalchemy.orm import Session
 
-from app.api.deps import get_db
+from app.api.deps import CurrentUserContext, get_db, require_admin_user
 from app.core.config import get_settings as get_runtime_settings
 from app.schemas.common import ErrorResponse
 from app.schemas.settings import (
     ModelConnectivityTestRead,
     ModelConnectivityTestRequest,
     RuntimeModelConfigRead,
+    SettingsCacheClearRead,
     SettingsRead,
     SettingsWrite,
 )
 from app.services.model_connectivity import probe_model_connectivity
 from app.services.settings import SettingsService
+from app.services.system_cache import SystemCacheService
 
 router = APIRouter(prefix="/settings")
 DbSession = Annotated[Session, Depends(get_db)]
@@ -93,6 +95,24 @@ def test_model_connectivity(
     return probe_model_connectivity(resolved_payload)
 
 
+@router.post(
+    "/cache/clear",
+    response_model=SettingsCacheClearRead,
+    summary="清理系统缓存",
+    description="清理 OCR、模型失败冷却、知识库索引和运行时配置等进程内缓存，不删除业务文件或数据库记录。",
+    responses={
+        status.HTTP_403_FORBIDDEN: {
+            "model": ErrorResponse,
+            "description": "只有管理员可以清理系统缓存。",
+        }
+    },
+)
+def clear_system_cache(
+    _: Annotated[CurrentUserContext, Depends(require_admin_user)],
+) -> SettingsCacheClearRead:
+    return SystemCacheService().clear_all()
+
+
 @router.get(
     "/runtime-models/{slot}",
     response_model=RuntimeModelConfigRead,