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.

app/composables/useBoardPosts.js

@@ -1,7 +1,3 @@
-/**
- * Board Posts Composable
- * Shared state + CRUD for board posts.
- */
 export function useBoardPosts() {
   const posts = useState('board.posts', () => [])
   const loading = useState('board.loading', () => false)
@@ -17,29 +13,29 @@ export function useBoardPosts() {
     }
   }
 
-  async function createPost(body, refreshParams = {}) {
+  async function createPost(body) {
     const created = await $fetch('/api/board/posts', {
       method: 'POST',
       body,
     })
-    await fetchPosts(refreshParams)
+    await fetchPosts()
     return created
   }
 
-  async function updatePost(id, body, refreshParams = {}) {
+  async function updatePost(id, body) {
     const updated = await $fetch(`/api/board/posts/${id}`, {
       method: 'PATCH',
       body,
     })
-    await fetchPosts(refreshParams)
+    await fetchPosts()
     return updated
   }
 
-  async function deletePost(id, refreshParams = {}) {
+  async function deletePost(id) {
     const result = await $fetch(`/api/board/posts/${id}`, {
       method: 'DELETE',
     })
-    await fetchPosts(refreshParams)
+    await fetchPosts()
     return result
   }