feat(config): 시스템 공통 설정 및 샘플 코드 관리

- SnpSystemConfig 엔티티/레포/서비스/컨트롤러 구현
- GET/PUT /api/config/{configKey} 엔드포인트
- 공통 샘플 코드 관리 admin 페이지 (SampleCodePage)
- 프론트엔드 configService 추가

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

frontend/src/pages/admin/SampleCodePage.tsx

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+import { useState, useEffect } from 'react';
+import { getSystemConfig, updateSystemConfig } from '../../services/configService';
+
+const COMMON_SAMPLE_CODE_KEY = 'COMMON_SAMPLE_CODE';
+
+const INPUT_CLS =
+  'w-full border border-gray-300 dark:border-gray-600 bg-white dark:bg-gray-700 text-gray-900 dark:text-gray-100 rounded-lg px-3 py-2 focus:ring-2 focus:ring-blue-500 focus:outline-none text-sm font-mono';
+
+const SampleCodePage = () => {
+  const [sampleCode, setSampleCode] = useState('');
+  const [loading, setLoading] = useState(true);
+  const [saving, setSaving] = useState(false);
+  const [message, setMessage] = useState<{ type: 'success' | 'error'; text: string } | null>(null);
+
+  useEffect(() => {
+    const fetchConfig = async () => {
+      try {
+        const res = await getSystemConfig(COMMON_SAMPLE_CODE_KEY);
+        if (res.success && res.data?.configValue != null) {
+          setSampleCode(res.data.configValue);
+        }
+      } catch {
+        setMessage({ type: 'error', text: '샘플 코드를 불러오는데 실패했습니다.' });
+      } finally {
+        setLoading(false);
+      }
+    };
+
+    fetchConfig();
+  }, []);
+
+  const handleSave = async () => {
+    setSaving(true);
+    setMessage(null);
+    try {
+      const res = await updateSystemConfig(COMMON_SAMPLE_CODE_KEY, sampleCode);
+      if (res.success) {
+        setMessage({ type: 'success', text: '저장되었습니다.' });
+      } else {
+        setMessage({ type: 'error', text: res.message || '저장에 실패했습니다.' });
+      }
+    } catch {
+      setMessage({ type: 'error', text: '저장 중 오류가 발생했습니다.' });
+    } finally {
+      setSaving(false);
+      setTimeout(() => setMessage(null), 3000);
+    }
+  };
+
+  if (loading) {
+    return (
+      <div className="flex items-center justify-center py-20">
+        <div className="w-8 h-8 border-4 border-blue-500 border-t-transparent rounded-full animate-spin" />
+      </div>
+    );
+  }
+
+  return (
+    <div className="max-w-4xl mx-auto">
+      <div className="flex items-start justify-between mb-6">
+        <div>
+          <h1 className="text-2xl font-bold text-gray-900 dark:text-gray-100">공통 샘플 코드 관리</h1>
+          <p className="text-sm text-gray-500 dark:text-gray-400 mt-1">
+            API HUB 상세 페이지에 공통으로 표시되는 샘플 코드를 관리합니다.
+          </p>
+        </div>
+        <button
+          onClick={handleSave}
+          disabled={saving}
+          className="px-4 py-2 rounded-lg bg-blue-600 hover:bg-blue-700 disabled:opacity-50 text-white text-sm font-medium transition-colors shrink-0"
+        >
+          {saving ? '저장 중...' : '저장'}
+        </button>
+      </div>
+
+      {message && (
+        <div
+          className={`mb-4 p-3 rounded-lg text-sm ${
+            message.type === 'success'
+              ? 'bg-green-50 dark:bg-green-900/30 text-green-700 dark:text-green-400'
+              : 'bg-red-50 dark:bg-red-900/30 text-red-700 dark:text-red-400'
+          }`}
+        >
+          {message.text}
+        </div>
+      )}
+
+      <div className="bg-white dark:bg-gray-800 rounded-lg shadow p-6">
+        <label className="block text-sm font-medium text-gray-700 dark:text-gray-300 mb-2">
+          샘플 코드
+        </label>
+        <textarea
+          value={sampleCode}
+          onChange={(e) => setSampleCode(e.target.value)}
+          rows={20}
+          placeholder="API HUB 상세 페이지에 표시할 공통 샘플 코드를 입력하세요."
+          className={`${INPUT_CLS} resize-y`}
+        />
+        <p className="mt-2 text-xs text-gray-400 dark:text-gray-500">
+          이 코드는 모든 API 상세 페이지의 &apos;요청 URL 생성&apos; 섹션 하단에 표시됩니다.
+        </p>
+      </div>
+    </div>
+  );
+};
+
+export default SampleCodePage;

frontend/src/services/configService.ts

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+import { get, put } from './apiClient';
+import type { SystemConfigInfo } from '../types/service';
+import type { ApiResponse } from '../types/api';
+
+export const getSystemConfig = (configKey: string): Promise<ApiResponse<SystemConfigInfo>> =>
+  get<SystemConfigInfo>(`/config/${configKey}`);
+
+export const updateSystemConfig = (
+  configKey: string,
+  configValue: string,
+): Promise<ApiResponse<SystemConfigInfo>> =>
+  put<SystemConfigInfo>(`/config/${configKey}`, { configValue });

src/main/java/com/gcsc/connection/common/controller/SystemConfigController.java

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+package com.gcsc.connection.common.controller;
+
+import com.gcsc.connection.common.dto.ApiResponse;
+import com.gcsc.connection.common.dto.SystemConfigResponse;
+import com.gcsc.connection.common.dto.UpdateSystemConfigRequest;
+import com.gcsc.connection.common.service.SystemConfigService;
+import lombok.RequiredArgsConstructor;
+import org.springframework.http.ResponseEntity;
+import org.springframework.web.bind.annotation.GetMapping;
+import org.springframework.web.bind.annotation.PathVariable;
+import org.springframework.web.bind.annotation.PutMapping;
+import org.springframework.web.bind.annotation.RequestBody;
+import org.springframework.web.bind.annotation.RequestMapping;
+import org.springframework.web.bind.annotation.RestController;
+
+/**
+ * 시스템 공통 설정 API
+ */
+@RestController
+@RequestMapping("/api/config")
+@RequiredArgsConstructor
+public class SystemConfigController {
+
+    private final SystemConfigService systemConfigService;
+
+    /**
+     * 설정 값 단건 조회
+     */
+    @GetMapping("/{configKey}")
+    public ResponseEntity<ApiResponse<SystemConfigResponse>> getConfig(@PathVariable String configKey) {
+        SystemConfigResponse response = systemConfigService.getConfigValue(configKey);
+        return ResponseEntity.ok(ApiResponse.ok(response));
+    }
+
+    /**
+     * 설정 값 저장 (upsert)
+     */
+    @PutMapping("/{configKey}")
+    public ResponseEntity<ApiResponse<SystemConfigResponse>> updateConfig(
+        @PathVariable String configKey,
+        @RequestBody UpdateSystemConfigRequest request
+    ) {
+        SystemConfigResponse response = systemConfigService.updateConfig(configKey, request);
+        return ResponseEntity.ok(ApiResponse.ok(response));
+    }
+}

src/main/java/com/gcsc/connection/common/dto/SystemConfigResponse.java

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+package com.gcsc.connection.common.dto;
+
+import com.gcsc.connection.common.entity.SnpSystemConfig;
+
+import java.time.LocalDateTime;
+
+public record SystemConfigResponse(
+    Long configId,
+    String configKey,
+    String configValue,
+    String description,
+    LocalDateTime createdAt,
+    LocalDateTime updatedAt
+) {
+
+    public static SystemConfigResponse from(SnpSystemConfig config) {
+        return new SystemConfigResponse(
+            config.getConfigId(),
+            config.getConfigKey(),
+            config.getConfigValue(),
+            config.getDescription(),
+            config.getCreatedAt(),
+            config.getUpdatedAt()
+        );
+    }
+}

src/main/java/com/gcsc/connection/common/dto/UpdateSystemConfigRequest.java

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+package com.gcsc.connection.common.dto;
+
+public record UpdateSystemConfigRequest(
+    String configValue
+) {
+}

src/main/java/com/gcsc/connection/common/entity/SnpSystemConfig.java

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+package com.gcsc.connection.common.entity;
+
+import jakarta.persistence.Column;
+import jakarta.persistence.Entity;
+import jakarta.persistence.GeneratedValue;
+import jakarta.persistence.GenerationType;
+import jakarta.persistence.Id;
+import jakarta.persistence.Table;
+import lombok.AccessLevel;
+import lombok.Builder;
+import lombok.Getter;
+import lombok.NoArgsConstructor;
+
+@Getter
+@NoArgsConstructor(access = AccessLevel.PROTECTED)
+@Entity
+@Table(name = "snp_system_config", schema = "common")
+public class SnpSystemConfig extends BaseEntity {
+
+    @Id
+    @GeneratedValue(strategy = GenerationType.IDENTITY)
+    @Column(name = "config_id")
+    private Long configId;
+
+    @Column(name = "config_key", unique = true, nullable = false, length = 100)
+    private String configKey;
+
+    @Column(name = "config_value", columnDefinition = "TEXT")
+    private String configValue;
+
+    @Column(name = "description", length = 500)
+    private String description;
+
+    @Builder
+    public SnpSystemConfig(String configKey, String configValue, String description) {
+        this.configKey = configKey;
+        this.configValue = configValue;
+        this.description = description;
+    }
+
+    public void update(String configValue) {
+        this.configValue = configValue;
+    }
+}

src/main/java/com/gcsc/connection/common/repository/SnpSystemConfigRepository.java

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+package com.gcsc.connection.common.repository;
+
+import com.gcsc.connection.common.entity.SnpSystemConfig;
+import org.springframework.data.jpa.repository.JpaRepository;
+
+import java.util.Optional;
+
+public interface SnpSystemConfigRepository extends JpaRepository<SnpSystemConfig, Long> {
+
+    Optional<SnpSystemConfig> findByConfigKey(String configKey);
+}

src/main/java/com/gcsc/connection/common/service/SystemConfigService.java

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+package com.gcsc.connection.common.service;
+
+import com.gcsc.connection.common.dto.SystemConfigResponse;
+import com.gcsc.connection.common.dto.UpdateSystemConfigRequest;
+import com.gcsc.connection.common.entity.SnpSystemConfig;
+import com.gcsc.connection.common.repository.SnpSystemConfigRepository;
+import lombok.RequiredArgsConstructor;
+import lombok.extern.slf4j.Slf4j;
+import org.springframework.stereotype.Service;
+import org.springframework.transaction.annotation.Transactional;
+
+@Slf4j
+@Service
+@RequiredArgsConstructor
+public class SystemConfigService {
+
+    private final SnpSystemConfigRepository systemConfigRepository;
+
+    /**
+     * 설정 값 단건 조회
+     */
+    @Transactional(readOnly = true)
+    public SystemConfigResponse getConfigValue(String configKey) {
+        return systemConfigRepository.findByConfigKey(configKey)
+            .map(SystemConfigResponse::from)
+            .orElse(null);
+    }
+
+    /**
+     * 설정 값 저장 (upsert: 없으면 생성, 있으면 수정)
+     */
+    @Transactional
+    public SystemConfigResponse updateConfig(String configKey, UpdateSystemConfigRequest request) {
+        SnpSystemConfig config = systemConfigRepository.findByConfigKey(configKey)
+            .map(existing -> {
+                existing.update(request.configValue());
+                return existing;
+            })
+            .orElseGet(() -> systemConfigRepository.save(
+                SnpSystemConfig.builder()
+                    .configKey(configKey)
+                    .configValue(request.configValue())
+                    .build()
+            ));
+        log.info("시스템 설정 저장: configKey={}", configKey);
+        return SystemConfigResponse.from(config);
+    }
+}