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/main.py

@@ -7,6 +7,7 @@ from app.api.departments import router as departments_router
 from app.api.spaces import router as spaces_router
 from app.api.projects import router as projects_router
 from app.api.tasks import router as tasks_router
+from app.api.workload import router as workload_router
 from app.core.config import settings
 
 app = FastAPI(
@@ -31,6 +32,7 @@ app.include_router(departments_router.router, prefix="/api/departments", tags=["
 app.include_router(spaces_router)
 app.include_router(projects_router)
 app.include_router(tasks_router)
+app.include_router(workload_router, prefix="/api/workload", tags=["Workload"])
 
 
 @app.get("/health")