fix: replace distroless Python 3.11 base with python:3.12-slim and add CI smoke test

[Copilot]

Production image crashes on startup because gcr.io/distroless/python3-debian12:nonroot ships Python 3.11.2 while the backend builder compiles against Python 3.12. Native extensions (pydantic_core._pydantic_core) fail to load.

ModuleNotFoundError: No module named 'pydantic_core._pydantic_core'

Dockerfile

• Replace distroless base with python:3.12-slim, matching the builder stage Python version
• Copy the full .venv and activate via PATH instead of manually setting PYTHONPATH to a version-specific site-packages path
• Add non-root user (chronowarden, uid 1000) and HEALTHCHECK instruction
• Install curl + ca-certificates for health checks

CI smoke test (docker.yaml)

• Build production image with load: true before any push
• Start the container and poll /api/v1/health for up to 30 seconds
• Fail the workflow if the health endpoint never responds, with verbose curl output and container logs for diagnostics
• Multi-platform build+push steps gated behind github.event_name != 'pull_request'

Original prompt

Fix production image. Just try to run it after building it, this will fail. The CI should have some kind of smoke test to just run and see if health endpoints on the backend are responding. Fail CI if container is unable to run after being built

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