refactor: 게시판별 태그 매핑 도입으로 불필요해진 board_setting 제거

- BoardSettingMapper(+XML)/BoardSettingRequest/Response 삭제
- TagService board_setting 의존 제거, /admin/board-setting 엔드포인트 제거
- board_setting 테이블 DDL 제거
- 글 수정 시 전체 그룹 조회(기존 글 태그 유실 방지): listWritableGroups(blank)=전체

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

src/main/java/com/sb/web/admin/controller/AdminTagController.java

@@ -1,7 +1,5 @@
 package com.sb.web.admin.controller;
 
-import com.sb.web.board.dto.BoardSettingRequest;
-import com.sb.web.board.dto.BoardSettingResponse;
 import com.sb.web.board.dto.TagCategoryRequest;
 import com.sb.web.board.dto.TagCategoryResponse;
 import com.sb.web.board.dto.TagRequest;
@@ -68,16 +66,4 @@ public class AdminTagController {
         tagService.deleteTag(id);
         return ResponseEntity.noContent().build();
     }
-
-    /* ===== 게시판 설정 (사용할 태그 카테고리) ===== */
-
-    @GetMapping("/board-setting")
-    public BoardSettingResponse boardSetting() {
-        return tagService.getBoardSetting();
-    }
-
-    @PutMapping("/board-setting")
-    public BoardSettingResponse updateBoardSetting(@RequestBody BoardSettingRequest req) {
-        return tagService.setBoardSetting(req.getTagCategoryId());
-    }
 }

src/main/java/com/sb/web/board/dto/BoardSettingRequest.java

@@ -1,11 +0,0 @@
-package com.sb.web.board.dto;
-
-import lombok.Data;
-
-/**
- * 게시판 설정 변경 요청 — 사용할 태그 카테고리(null 이면 제한 없음).
- */
-@Data
-public class BoardSettingRequest {
-    private Long tagCategoryId;
-}

src/main/java/com/sb/web/board/dto/BoardSettingResponse.java

@@ -1,13 +0,0 @@
-package com.sb.web.board.dto;
-
-import lombok.Builder;
-import lombok.Data;
-
-/**
- * 게시판 설정 응답.
- */
-@Data
-@Builder
-public class BoardSettingResponse {
-    private Long tagCategoryId;
-}

src/main/java/com/sb/web/board/mapper/BoardSettingMapper.java

@@ -1,13 +0,0 @@
-package com.sb.web.board.mapper;
-
-import org.apache.ibatis.annotations.Mapper;
-import org.apache.ibatis.annotations.Param;
-
-@Mapper
-public interface BoardSettingMapper {
-
-    /** 게시판에서 사용할 태그 카테고리 ID (없으면 null) */
-    Long findTagCategoryId();
-
-    int updateTagCategoryId(@Param("categoryId") Long categoryId);
-}

src/main/java/com/sb/web/board/service/TagService.java

@@ -4,10 +4,8 @@ import com.sb.web.board.domain.Tag;
 import com.sb.web.board.domain.TagCategory;
 import com.sb.web.board.dto.TagCategoryRequest;
 import com.sb.web.board.dto.TagCategoryResponse;
-import com.sb.web.board.dto.BoardSettingResponse;
 import com.sb.web.board.dto.TagRequest;
 import com.sb.web.board.dto.TagResponse;
-import com.sb.web.board.mapper.BoardSettingMapper;
 import com.sb.web.board.mapper.TagCategoryBoardMapper;
 import com.sb.web.board.mapper.TagCategoryMapper;
 import com.sb.web.board.mapper.TagMapper;
@@ -30,7 +28,6 @@ public class TagService {
 
     private final TagCategoryMapper categoryMapper;
     private final TagMapper tagMapper;
-    private final BoardSettingMapper boardSettingMapper;
     private final TagCategoryBoardMapper categoryBoardMapper;
 
     /** 게시판 매핑에 허용되는 값 */
@@ -50,6 +47,10 @@ public class TagService {
      * 매핑이 하나도 없는 그룹은 전체 게시판에서 노출(하위호환).
      */
     public List<TagCategoryResponse> listWritableGroups(String boardCategory) {
+        // 게시판 미지정(수정 화면 등) → 전체 그룹 (기존 글 태그 유실 방지)
+        if (boardCategory == null || boardCategory.isBlank()) {
+            return listGrouped();
+        }
         List<TagCategoryResponse> result = new ArrayList<>();
         for (TagCategory c : categoryMapper.findAll()) {
             TagCategoryResponse g = toGroup(c);
@@ -80,23 +81,6 @@ public class TagService {
                 .forEach(b -> categoryBoardMapper.insert(categoryId, b));
     }
 
-    /* ===================== 게시판 설정 ===================== */
-
-    public BoardSettingResponse getBoardSetting() {
-        return BoardSettingResponse.builder()
-                .tagCategoryId(boardSettingMapper.findTagCategoryId())
-                .build();
-    }
-
-    @Transactional
-    public BoardSettingResponse setBoardSetting(Long tagCategoryId) {
-        if (tagCategoryId != null && categoryMapper.findById(tagCategoryId) == null) {
-            throw new ApiException(HttpStatus.NOT_FOUND, "카테고리를 찾을 수 없습니다.");
-        }
-        boardSettingMapper.updateTagCategoryId(tagCategoryId);
-        return BoardSettingResponse.builder().tagCategoryId(tagCategoryId).build();
-    }
-
     /* ===================== 카테고리 ===================== */
 
     @Transactional