Fix po-email-processor bundling for arm64 target (#34)

po-email-processor has been down since 2026-05-12: the bundling
command installed wheels for the build host's architecture, so CD
deploys from amd64 runners shipped x86_64 pydantic_core into an
ARM_64 function, crashing every INIT with Runtime.ImportModuleError.

Pin the pip platform to manylinux2014_aarch64 with --only-binary,
matching the pattern wo_stack already uses.

Verified live: deployed from branch, manual invoke OK; 920 emails
from the outage window backfilled via scripts/reprocess logic.

cdk/po_stack.py

@@ -73,7 +73,9 @@ class PoIngestStack(Stack):
                     command=[
                         "bash",
                         "-c",
-                        "pip install -r requirements.txt -t /asset-output && cp handler.py /asset-output/",
+                        "pip install --platform manylinux2014_aarch64 --only-binary=:all: "
+                        "-r requirements.txt -t /asset-output && "
+                        "cp handler.py /asset-output/",
                     ],
                 ),
             ),