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/member.js

@@ -73,17 +73,7 @@ const memberSchema = new mongoose.Schema({
 
   craftTags: [String],
   board: {
-    topics: [
-      {
-        tagSlug: String,
-        state: { type: String, enum: ['help', 'interested', 'seeking'] },
-      },
-    ],
-    offerPeerSupport: { type: Boolean, default: false },
-    availability: String,
     slackHandle: String,
-    personalMessage: String,
-    details: String,
   },
 
   // Privacy settings for profile fields