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/migrations/versions/003_workload_snapshots_table.py

@@ -0,0 +1,38 @@
+"""Workload snapshots table
+
+Revision ID: 003
+Revises: 002
+Create Date: 2024-01-XX
+
+"""
+from alembic import op
+import sqlalchemy as sa
+
+# revision identifiers, used by Alembic.
+revision = '003'
+down_revision = '002'
+branch_labels = None
+depends_on = None
+
+
+def upgrade() -> None:
+    # Create pjctrl_workload_snapshots table
+    op.create_table(
+        'pjctrl_workload_snapshots',
+        sa.Column('id', sa.String(36), primary_key=True),
+        sa.Column('user_id', sa.String(36), sa.ForeignKey('pjctrl_users.id', ondelete='CASCADE'), nullable=False),
+        sa.Column('week_start', sa.Date, nullable=False),
+        sa.Column('allocated_hours', sa.Numeric(8, 2), nullable=False, server_default='0'),
+        sa.Column('capacity_hours', sa.Numeric(8, 2), nullable=False, server_default='40'),
+        sa.Column('load_percentage', sa.Numeric(5, 2), nullable=False, server_default='0'),
+        sa.Column('task_count', sa.Integer, nullable=False, server_default='0'),
+        sa.Column('created_at', sa.DateTime, server_default=sa.func.now(), nullable=False),
+        sa.Column('updated_at', sa.DateTime, server_default=sa.func.now(), onupdate=sa.func.now(), nullable=False),
+        sa.UniqueConstraint('user_id', 'week_start', name='uk_user_week'),
+    )
+    op.create_index('idx_workload_user', 'pjctrl_workload_snapshots', ['user_id'])
+    op.create_index('idx_workload_week_start', 'pjctrl_workload_snapshots', ['week_start'])
+
+
+def downgrade() -> None:
+    op.drop_table('pjctrl_workload_snapshots')