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/tests/conftest.py

@@ -29,6 +29,9 @@ class MockRedis:
     def get(self, key):
         return self.store.get(key)
 
+    def set(self, key, value):
+        self.store[key] = value
+
     def setex(self, key, seconds, value):
         self.store[key] = value
 
@@ -36,6 +39,17 @@ class MockRedis:
         if key in self.store:
             del self.store[key]
 
+    def scan_iter(self, match=None):
+        """Iterate over keys matching a pattern."""
+        import fnmatch
+        if match is None:
+            yield from self.store.keys()
+        else:
+            pattern = match.replace("*", "**")
+            for key in self.store.keys():
+                if fnmatch.fnmatch(key, match):
+                    yield key
+
 
 @pytest.fixture(scope="function")
 def db():