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/boardPost.js

@@ -0,0 +1,28 @@
+import mongoose from 'mongoose'
+
+const boardPostSchema = new mongoose.Schema({
+  author: {
+    type: mongoose.Schema.Types.ObjectId,
+    ref: 'Member',
+    required: true,
+  },
+  title: { type: String, required: true, maxlength: 120 },
+  seeking: { type: String, maxlength: 500 },
+  offering: { type: String, maxlength: 500 },
+  note: { type: String, maxlength: 300 },
+  tags: [String],
+}, { timestamps: true })
+
+boardPostSchema.pre('validate', function (next) {
+  const seeking = (this.seeking || '').trim()
+  const offering = (this.offering || '').trim()
+  if (!seeking && !offering) {
+    this.invalidate('seeking', 'At least one of seeking or offering must be provided')
+  }
+  next()
+})
+
+boardPostSchema.index({ author: 1 })
+boardPostSchema.index({ createdAt: -1 })
+
+export default mongoose.models.BoardPost || mongoose.model('BoardPost', boardPostSchema)