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/utils/activityLog.js

@@ -18,6 +18,7 @@ export const ACTIVITY_TYPES = {
   EMAIL_SENT: 'email_sent',
   COMMUNITY_CONNECTIONS_UPDATED: 'community_connections_updated',
   BOARD_UPDATED: 'board_updated',
+  BOARD_POST_CREATED: 'board_post_created',
   CONNECTION_REQUESTED: 'connection_requested',
   CONNECTION_CONFIRMED: 'connection_confirmed',
   TAG_SUGGESTED: 'tag_suggested'
@@ -41,6 +42,7 @@ export const ACTIVITY_TYPE_DEFAULTS = {
   email_sent: 'member',
   community_connections_updated: 'member',
   board_updated: 'member',
+  board_post_created: 'member',
   connection_requested: 'member',
   connection_confirmed: 'member',
   tag_suggested: 'member'