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/components/BoardPostCard.vue

@@ -86,6 +86,8 @@ const props = defineProps({
 
 defineEmits(['edit', 'delete', 'confirm-delete', 'cancel-delete'])
 
+const { slackUrl } = useBoardChannels()
+
 const capitalizeAvatar = (str) => {
   if (str.toLowerCase() === 'wtf') return 'WTF'
   return str
@@ -148,7 +150,7 @@ const slackLinks = computed(() => {
     .map((c) => ({
       id: c.slackChannelId,
       name: c.slackChannelName || c.name || c.slackChannelId,
-      url: `https://gammaspace.slack.com/archives/${c.slackChannelId}`,
+      url: slackUrl(c.slackChannelId),
     }))
 })
 </script>