refactor(board): atomic delete + query limit + composable cleanup

Delete uses findOneAndDelete with author match (no TOCTOU window);
existence check only runs on miss to distinguish 403 vs 404. Posts
list capped at 200. Drop unused resolveTagChannel and refreshParams;
route slack URL building through the composable's slackUrl helper.

server/api/board/posts.get.js

@@ -17,6 +17,7 @@ export default defineEventHandler(async (event) => {
 
   const posts = await BoardPost.find(dbQuery)
     .sort({ createdAt: -1 })
+    .limit(200)
     .populate('author', 'name avatar circle board.slackHandle')
     .lean()