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/channels.get.js

@@ -0,0 +1,10 @@
+import BoardChannel from '../../models/boardChannel.js'
+import { requireAuth } from '../../utils/auth.js'
+
+export default defineEventHandler(async (event) => {
+  await requireAuth(event)
+
+  const channels = await BoardChannel.find({}).sort({ name: 1 }).lean()
+
+  return { channels }
+})