fix(backend): expose c2_task.source in GET /c2/tasks response

Serialize source as t.source.value (string) in list_simulation_tasks.
Updated test_c2_tasks_list shape assertion to include 'source' and
assert value is 'mimic' for execute-created tasks. Added test in
test_c2_import to assert source='import' in GET /c2/tasks after import.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/tests/test_c2_import.py

@@ -282,6 +282,25 @@ class TestImportHappyPath:
         resp = _import(client, redteam_token, sim["id"], [100])
         assert resp.status_code == 200
 
+    def test_source_field_is_import_in_tasks_listing(
+        self, monkeypatch, client: FlaskClient, admin_token: str
+    ) -> None:
+        """Imported tasks appear with source='import' in GET /c2/tasks response."""
+        _make_completed_get_task(monkeypatch)
+        eng = _make_engagement(client, admin_token)
+        _put_config(client, admin_token, eng["id"])
+        sim = _make_sim(client, admin_token, eng["id"])
+
+        _import(client, admin_token, sim["id"], [100])
+
+        resp = client.get(
+            f"/api/simulations/{sim['id']}/c2/tasks",
+            headers=_h(admin_token),
+        )
+        assert resp.status_code == 200
+        task = resp.get_json()["tasks"][0]
+        assert task["source"] == "import"
+
     def test_no_transition_when_all_skipped(
         self, app: Flask, monkeypatch, client: FlaskClient, admin_token: str
     ) -> None: