- 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
