feat: implement workload heatmap module

- Backend (FastAPI):
  - Workload heatmap API with load level calculation
  - User workload detail endpoint with task breakdown
  - Redis caching for workload calculations (1hr TTL)
  - Department isolation and access control
  - WorkloadSnapshot model for historical data
  - Alembic migration for workload_snapshots table

- API Endpoints:
  - GET /api/workload/heatmap - Team workload overview
  - GET /api/workload/user/{id} - User workload detail
  - GET /api/workload/me - Current user workload

- Load Levels:
  - normal: <80%, warning: 80-99%, overloaded: >=100%

- Tests:
  - 26 unit/API tests
  - 15 E2E automated tests
  - 77 total tests passing

- OpenSpec:
  - add-resource-workload change archived
  - resource-management spec updated

🤖 Generated with [Claude Code](https://claude.com/claude-code)

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

backend/app/models/__init__.py

@@ -5,5 +5,6 @@ from app.models.space import Space
 from app.models.project import Project
 from app.models.task_status import TaskStatus
 from app.models.task import Task
+from app.models.workload_snapshot import WorkloadSnapshot
 
-__all__ = ["User", "Role", "Department", "Space", "Project", "TaskStatus", "Task"]
+__all__ = ["User", "Role", "Department", "Space", "Project", "TaskStatus", "Task", "WorkloadSnapshot"]