feat: 태그 그룹을 게시판별로 매핑 — 게시판 성격에 맞는 태그만 노출

- tag_category_board 매핑 테이블 추가(그룹 ↔ community/saving/tips)
- TagCategory 응답에 boards 추가, 생성/수정 시 매핑 저장(유효 게시판만)
- listWritableGroups(category): 해당 게시판에 매핑된 그룹만(매핑 없으면 전체=하위호환)
- GET /board/tag-groups?category=... 로 게시판별 필터

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

src/main/resources/mapper/TagCategoryBoardMapper.xml

@@ -0,0 +1,19 @@
+<?xml version="1.0" encoding="UTF-8"?>
+<!DOCTYPE mapper PUBLIC "-//mybatis.org//DTD Mapper 3.0//EN"
+        "https://mybatis.org/dtd/mybatis-3-mapper.dtd">
+<mapper namespace="com.sb.web.board.mapper.TagCategoryBoardMapper">
+
+    <select id="findBoards" parameterType="long" resultType="string">
+        SELECT board_category FROM tag_category_board WHERE tag_category_id = #{categoryId}
+    </select>
+
+    <delete id="deleteByCategory" parameterType="long">
+        DELETE FROM tag_category_board WHERE tag_category_id = #{categoryId}
+    </delete>
+
+    <insert id="insert">
+        INSERT INTO tag_category_board (tag_category_id, board_category)
+        VALUES (#{categoryId}, #{board})
+    </insert>
+
+</mapper>