feat(backend): 新增公共配置读取接口 PublicConfigController

- POST /api/config/public/value 根据 key 返回系统配置值
- 路径已在 WebConfig JWT 白名单（/api/config/public/**），免登录访问

cfc-backend/src/main/java/com/etotem/cfc/controller/config/PublicConfigController.java

@@ -0,0 +1,29 @@
+package com.etotem.cfc.controller.config;
+
+import com.etotem.cfc.common.Result;
+import com.etotem.cfc.service.SysConfigService;
+import org.springframework.web.bind.annotation.PostMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+import javax.annotation.Resource;
+import java.util.Map;
+
+@RestController
+@RequestMapping("/api/config/public")
+public class PublicConfigController {
+
+    @Resource
+    private SysConfigService configService;
+
+    @PostMapping("/value")
+    public Result<String> getValue(@RequestBody Map<String, String> params) {
+        String key = params.get("key");
+        if (key == null || key.isEmpty()) {
+            return Result.error("key is required");
+        }
+        String value = configService.getValue(key);
+        return Result.success(value);
+    }
+}