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/admin/board-channels/[id].delete.js

@@ -0,0 +1,14 @@
+import BoardChannel from '../../../models/boardChannel.js'
+import { requireAdmin } from '../../../utils/auth.js'
+
+export default defineEventHandler(async (event) => {
+  await requireAdmin(event)
+  const id = getRouterParam(event, 'id')
+
+  const channel = await BoardChannel.findByIdAndDelete(id)
+  if (!channel) {
+    throw createError({ statusCode: 404, statusMessage: 'Channel not found' })
+  }
+
+  return { success: true }
+})