feat: 게시판 공지 기능 (관리자) — 목록 최상단 고정

- post.notice 컬럼 추가(멱등 ALTER)
- 목록 정렬 ORDER BY notice DESC, id DESC (공지 최상단)
- 작성 시 공지 설정(관리자만 반영) + /posts/{id}/notice·unnotice 토글
- Post/PostSummary/PostDetail/PostRequest 에 notice 반영

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

src/main/resources/mapper/PostMapper.xml

@@ -35,11 +35,11 @@
 
     <select id="findPage" resultType="com.sb.web.board.dto.PostSummary">
         SELECT
-            p.id, p.title, p.author_name, p.view_count, p.blocked, p.created_at,
+            p.id, p.title, p.author_name, p.view_count, p.blocked, p.notice, p.created_at,
             (SELECT COUNT(*) FROM comment c WHERE c.post_id = p.id) AS comment_count
         FROM post p
         <include refid="filter"/>
-        ORDER BY p.id DESC
+        ORDER BY p.notice DESC, p.id DESC
         LIMIT #{size} OFFSET #{offset}
     </select>
 
@@ -51,15 +51,15 @@
 
     <select id="findById" parameterType="long" resultType="com.sb.web.board.domain.Post">
         SELECT id, title, content, author_id, author_name, view_count,
-               blocked, block_reason, category, created_at, updated_at
+               blocked, block_reason, notice, category, created_at, updated_at
         FROM post
         WHERE id = #{id}
     </select>
 
     <insert id="insert" parameterType="com.sb.web.board.domain.Post"
             useGeneratedKeys="true" keyProperty="id">
-        INSERT INTO post (title, content, author_id, author_name, view_count, category, created_at, updated_at)
-        VALUES (#{title}, #{content}, #{authorId}, #{authorName}, 0, #{category}, NOW(), NOW())
+        INSERT INTO post (title, content, author_id, author_name, view_count, notice, category, created_at, updated_at)
+        VALUES (#{title}, #{content}, #{authorId}, #{authorName}, 0, #{notice}, #{category}, NOW(), NOW())
     </insert>
 
     <update id="update" parameterType="com.sb.web.board.domain.Post">
@@ -80,4 +80,8 @@
         UPDATE post SET blocked = #{blocked}, block_reason = #{blockReason} WHERE id = #{id}
     </update>
 
+    <update id="updateNotice">
+        UPDATE post SET notice = #{notice} WHERE id = #{id}
+    </update>
+
 </mapper>