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/pages/board.vue

@@ -1,13 +1,11 @@
 <template>
   <PageShell title="Bulletin Board" :subtitle="pageSubtitle">
-    <!-- Action bar -->
     <div class="action-bar">
       <button type="button" class="new-post-btn" @click="openNewForm">
         + New Post
       </button>
     </div>
 
-    <!-- Tags Drawer Toggle -->
     <div v-if="cooperativeTags.length > 0" class="tags-drawer-toggle">
       <button type="button" class="drawer-btn" @click="showTagsDrawer = !showTagsDrawer">
         Tags...
@@ -15,7 +13,6 @@
       </button>
     </div>
 
-    <!-- Tags Drawer -->
     <div v-if="showTagsDrawer && cooperativeTags.length > 0" class="tags-drawer">
       <div class="skills-bar">
         <span class="tag-label">Filter:</span>
@@ -40,7 +37,6 @@
       </div>
     </div>
 
-    <!-- Inline form -->
     <div v-if="showForm" class="form-wrapper">
       <BoardPostForm
         :post="editingPost"
@@ -50,7 +46,6 @@
       />
     </div>
 
-    <!-- Content -->
     <ClientOnly>
       <div v-if="loading" class="loading-state">
         <p>Loading board...</p>