chore(deps): update dependency mangum to v0.21.0 (#543)

Co-authored-by: renovate[bot] <29139614+renovate[bot]@users.noreply.github.com>

Author: renovate[bot]

api/requirements.txt

@@ -1,7 +1,7 @@
 boto3==1.42.39
 cryptography==46.0.4
 fastapi==0.128.0
-mangum==0.20.0
+mangum==0.21.0
 python-dotenv==1.2.1
 requests==2.32.5
 urllib3==2.6.3