fix(board-channels): dev stub slackChannelId must match update schema

The ALLOW_DEV_TEST_ENDPOINTS short-circuit on create wrote
'dev-stub-<ms>' as the channel ID. boardChannelUpdateSchema requires
^[A-Z0-9]+$, so the very next edit on the same channel hit a 400 from
Zod and the table never updated. Use base36-uppercased timestamp with
a 'CDEV' prefix so the stub survives a round-trip through the patch
route. Live path is unchanged.

server/api/admin/board-channels.post.js

@@ -25,7 +25,9 @@ export default defineEventHandler(async (event) => {
 
   if (!slackChannelId) {
     if (process.env.ALLOW_DEV_TEST_ENDPOINTS === 'true') {
-      slackChannelId = `dev-stub-${Date.now()}`
+      // Match the Slack channel ID format (^[A-Z0-9]+$) so the value
+      // round-trips through boardChannelUpdateSchema on subsequent edits.
+      slackChannelId = `CDEV${Date.now().toString(36).toUpperCase()}`
       console.log('[slack] DEV MODE — skipping createChannel', { name: body.name, slackChannelId })
     } else {
       const slack = getSlackAdminService()