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/workload_snapshot.py

@@ -0,0 +1,29 @@
+import uuid
+from sqlalchemy import Column, String, ForeignKey, Date, Integer, Numeric, DateTime, UniqueConstraint, Index
+from sqlalchemy.sql import func
+from sqlalchemy.orm import relationship
+from app.core.database import Base
+
+
+class WorkloadSnapshot(Base):
+    """Stores historical workload snapshots for trend analysis."""
+    __tablename__ = "pjctrl_workload_snapshots"
+
+    id = Column(String(36), primary_key=True, default=lambda: str(uuid.uuid4()))
+    user_id = Column(String(36), ForeignKey("pjctrl_users.id", ondelete="CASCADE"), nullable=False)
+    week_start = Column(Date, nullable=False)
+    allocated_hours = Column(Numeric(8, 2), nullable=False, default=0)
+    capacity_hours = Column(Numeric(8, 2), nullable=False, default=40)
+    load_percentage = Column(Numeric(5, 2), nullable=False, default=0)
+    task_count = Column(Integer, nullable=False, default=0)
+    created_at = Column(DateTime, server_default=func.now(), nullable=False)
+    updated_at = Column(DateTime, server_default=func.now(), onupdate=func.now(), nullable=False)
+
+    # Relationships
+    user = relationship("User", backref="workload_snapshots")
+
+    # Constraints
+    __table_args__ = (
+        UniqueConstraint('user_id', 'week_start', name='uk_user_week'),
+        Index('idx_workload_week_start', 'week_start'),
+    )