BE-PR-006: Add Essential CRUD API under Proposals

- New router: /projects/{project_id}/proposals/{proposal_id}/essentials
  - GET (list), POST (create), GET/{id}, PATCH/{id}, DELETE/{id}
- All mutations restricted to open proposals only
- Permission: creator, project owner, or global admin
- Registered essentials router in main.py
- Updated GET /proposals/{id} to return ProposalDetailResponse with
  embedded essentials list
- Activity logging on all CRUD operations

app/main.py

@@ -61,6 +61,7 @@ from app.api.routers.proposals import router as proposals_router
 from app.api.routers.proposes import router as proposes_router  # legacy compat
 from app.api.routers.milestone_actions import router as milestone_actions_router
 from app.api.routers.meetings import router as meetings_router
+from app.api.routers.essentials import router as essentials_router
 
 app.include_router(auth_router)
 app.include_router(tasks_router)
@@ -76,6 +77,7 @@ app.include_router(proposals_router)
 app.include_router(proposes_router)  # legacy compat
 app.include_router(milestone_actions_router)
 app.include_router(meetings_router)
+app.include_router(essentials_router)
 
 
 # Auto schema migration for lightweight deployments