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/[id].delete.js

@@ -0,0 +1,20 @@
+import BoardPost from '../../../models/boardPost.js'
+import { requireAuth } from '../../../utils/auth.js'
+
+export default defineEventHandler(async (event) => {
+  const member = await requireAuth(event)
+  const id = getRouterParam(event, 'id')
+
+  const post = await BoardPost.findById(id)
+  if (!post) {
+    throw createError({ statusCode: 404, statusMessage: 'Post not found' })
+  }
+
+  if (post.author.toString() !== member._id.toString()) {
+    throw createError({ statusCode: 403, statusMessage: 'Not authorized to delete this post' })
+  }
+
+  await post.deleteOne()
+
+  return { success: true }
+})