feat: board post + channel API routes

Implements Phase 2a of board classifieds redesign:

- GET/POST /api/board/posts (list with tag/author filters, create)
- PATCH/DELETE /api/board/posts/:id (author-only)
- GET /api/board/channels (member)
- POST /api/admin/board-channels (admin)
- PATCH/DELETE /api/admin/board-channels/:id (admin)

Adds board_post_created activity type.

server/api/board/posts.post.js

@@ -0,0 +1,28 @@
+import BoardPost from '../../models/boardPost.js'
+import { requireAuth } from '../../utils/auth.js'
+import { validateBody } from '../../utils/validateBody.js'
+import { boardPostCreateSchema } from '../../utils/schemas.js'
+import { logActivity, ACTIVITY_TYPES } from '../../utils/activityLog.js'
+
+export default defineEventHandler(async (event) => {
+  const member = await requireAuth(event)
+
+  const body = await validateBody(event, boardPostCreateSchema)
+
+  const post = new BoardPost({
+    author: member._id,
+    title: body.title,
+    seeking: body.seeking,
+    offering: body.offering,
+    note: body.note,
+    tags: body.tags || []
+  })
+
+  await post.save()
+  await post.populate('author', 'name avatar circle board.slackHandle')
+
+  logActivity(member._id, ACTIVITY_TYPES.BOARD_POST_CREATED, { postId: post._id, title: post.title })
+
+  setResponseStatus(event, 201)
+  return { post: post.toObject() }
+})