Release pipeline: SBOM, image signing (cosign) and container scanning

[windischb]

The release pipeline (cd-release.yml) builds and publishes multi-arch images and NuGet packages but produces no supply-chain trust artifacts yet:

• No SBOM generation (e.g. syft) attached to releases
• No image/artifact signing or provenance attestations (cosign/sigstore, GitHub artifact attestations)
• No container image scanning (Trivy/Grype) as a release gate

For an identity product these are high-leverage, comparatively cheap trust signals. Suggested order: SBOM → attestations/signing → image scanning.