feat: meetings code-based API + TODO updates

- Backend: added meetings router with code-first CRUD + attend/participant model
- TODO: mark meeting-code API and attend route as completed

plans/hf-cross-project-todo.md

@@ -204,7 +204,7 @@
 - [x] Ensure `project-code` is accepted in get/update/delete/member APIs
 - [x] Ensure `milestone-code` is accepted in get/update/delete/progress APIs
 - [x] Ensure `task-code` is accepted in get/update/delete/transition APIs
-- [ ] Ensure `meeting-code` is accepted in get/update/delete/attend APIs
+- [x] Ensure `meeting-code` is accepted in get/update/delete/attend APIs
 - [x] Ensure `support-code` is accepted in get/update/delete/take/transition APIs
 - [x] Ensure `propose-code` is accepted in get/update/delete/accept/reject/reopen APIs
 - [x] Add code-based list filters where missing
@@ -226,8 +226,8 @@
 ### 2.4 Task / meeting / support actions
 - [x] Add/confirm backend route for task self-assignment (`task take`)
 - [x] Add/confirm conflict behavior when task already has a different assignee
-- [ ] Add/confirm backend route for meeting attendance (`meeting attend`)
-- [ ] Ensure meeting attendance updates participant list
+- [x] Add/confirm backend route for meeting attendance (`meeting attend`)
+- [x] Ensure meeting attendance updates participant list
 - [x] Add/confirm backend route for support self-assignment (`support take`)
 
 ### 2.5 Frontend-facing code migration support