feat(board): add BoardPost + BoardChannel models and zod schemas

- Add BoardPost model (author, title, seeking/offering, note, tags) with
  validator requiring at least one of seeking/offering
- Add BoardChannel model (name, slackChannelId, tagSlugs)
- Add boardPost/boardChannel create+update Zod schemas
- Trim Member.board subdoc to only slackHandle (drop topics, details,
  offerPeerSupport, availability, personalMessage)
- Remove old boardUpdateSchema

server/models/boardChannel.js

@@ -0,0 +1,9 @@
+import mongoose from 'mongoose'
+
+const boardChannelSchema = new mongoose.Schema({
+  name: { type: String, required: true },
+  slackChannelId: { type: String, required: true },
+  tagSlugs: [String],
+}, { timestamps: true })
+
+export default mongoose.models.BoardChannel || mongoose.model('BoardChannel', boardChannelSchema)