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/useBoardChannels.js

@@ -1,7 +1,3 @@
-/**
- * Board Channels Composable
- * Shared state + helpers for mapping board tags to Slack channels.
- */
 export function useBoardChannels() {
   const channels = useState('board.channels', () => [])
 
@@ -11,15 +7,6 @@ export function useBoardChannels() {
     return channels.value
   }
 
-  function resolveTagChannel(tagSlugs = []) {
-    if (!tagSlugs?.length) return null
-    return (
-      channels.value.find((channel) =>
-        (channel.tagSlugs || []).some((slug) => tagSlugs.includes(slug))
-      ) || null
-    )
-  }
-
   function slackUrl(channelId) {
     return `https://gammaspace.slack.com/archives/${channelId}`
   }
@@ -27,7 +14,6 @@ export function useBoardChannels() {
   return {
     channels: readonly(channels),
     fetchChannels,
-    resolveTagChannel,
     slackUrl,
   }
 }