Fix uv segfault during cross-platform Docker build

[Cherubeam]

Summary

• Split Python dependency resolution into a --platform=$BUILDPLATFORM stage that runs uv export --frozen natively, avoiding the QEMU segfault when cross-building linux/amd64 from an arm64 host.
• Install the pre-resolved requirements with pip in the runtime stage, since any uv execution under QEMU still crashes. uv remains in the image so CMD ["uv", "run", ...] works on Azure Container Apps / Cloud Run.
• Drop the separate uv tool install semgrep — semgrep is already a regular dependency in backend/pyproject.toml, so its CLI lands via the normal install.

Why

terraform apply was failing with uv sync --frozen returning exit 139 (SIGSEGV) on Apple Silicon hosts, because the terraform Docker provider cross-builds linux/amd64 under QEMU and uv's resolver crashes under emulation.

Test plan

• docker build --platform linux/amd64 -t cyber-analyzer:verify . succeeds on Apple Silicon
• terraform apply in terraform/azure completes and the deployed container responds on /health
• terraform apply in terraform/gcp (once GCP is set up) does the same