The .NET API Lambda Function URL uses authType=AWS_IAM, but the four workload
Lambdas (suggestions, pdf-extract, pdf-generate, library-ingest) sent unsigned
requests with only X-Internal-Api-Key -> every internal call 403s. They also
used bare fromAsset() with no pip bundling -> ImportError at cold start. Both
made the SQS->Lambda->API pipeline non-functional when deployed (v1 pre-flight).
- Add _sign_request_headers (botocore SigV4Auth, service "lambda"); serialize the
JSON body once and send via httpx content= so the signed payload hash matches
the bytes sent; preserve X-Internal-Api-Key for the app-layer check. Sign per
retry attempt to avoid SigV4 timestamp expiry on slow retries.
- Add CDK pip bundling (--platform manylinux2014_aarch64 --only-binary=:all:) to
all four Lambdas so ARM64 wheels (reportlab, Pillow, pdfplumber) ship.
- Converge _retry_request across all four (fixes possibly-undefined return in
pdf-extract/pdf-generate).
- Add SigV4 signing regression tests.
Verified: ruff clean, infra tsc clean, aarch64 wheels resolve for all four,
23 pytest pass. GPT-4.1 cross-family review: no BLOCK (FIX + NIT applied).