fix: export restorable CSV backups

tests/test_app.py

@@ -359,6 +359,18 @@ def test_restore_replace_recovers_habits_and_task_links_without_tags(client):
     assert "tags" not in exported.json()
     assert "task_tags" not in exported.json()
 
+    exported_csv = client.get("/api/v1/export.csv")
+    assert exported_csv.status_code == 200
+    assert exported_csv.headers["content-type"].startswith("text/csv")
+    assert "dodo-export.csv" in exported_csv.headers["content-disposition"]
+    assert exported_csv.content.startswith(b"\xef\xbb\xbfentity,data")
+
+    restored_csv = client.post(
+        "/api/v1/restore.csv?mode=merge",
+        files={"file": ("dodo-export.csv", exported_csv.content, "text/csv")},
+    )
+    assert restored_csv.status_code == 200
+
     client.post(
         "/api/v1/habits",
         json={"name": "深蹲", "kind": "boolean", "schedule_type": "daily"},