feat: add code-based meetings router with participant/attend support

- New dedicated meetings.py router with full CRUD (list/get/create/update/delete)
- All endpoints accept meeting_code or numeric id
- MeetingParticipant model for tracking meeting attendance
- POST /meetings/{id}/attend adds current user to participant list
- Serialization includes participants list, project_code, milestone_code
- Creator auto-added as participant on meeting creation
- Registered in main.py alongside existing routers

app/main.py

@@ -39,6 +39,7 @@ from app.api.routers.milestones import router as milestones_router
 from app.api.routers.roles import router as roles_router
 from app.api.routers.proposes import router as proposes_router
 from app.api.routers.milestone_actions import router as milestone_actions_router
+from app.api.routers.meetings import router as meetings_router
 
 app.include_router(auth_router)
 app.include_router(tasks_router)
@@ -52,6 +53,7 @@ app.include_router(milestones_router)
 app.include_router(roles_router)
 app.include_router(proposes_router)
 app.include_router(milestone_actions_router)
+app.include_router(meetings_router)
 
 
 # Auto schema migration for lightweight deployments