Pin base images to immutable sha256 digests for reproducible and more secure builds. Updated ARG BASE_VERSION to include digest in docker/backend/Dockerfile, docker/embedded/Dockerfile, and docker/embedded/Dockerfile.fat; replaced eclipse-temurin base in embedded/Dockerfile.fat with a sha256-pinned reference; and pinned the ghcr.io/astral-sh/uv base image in engine/Dockerfile.dev.
# Description of Changes
Redesign the Python AI engine to be properly agentic and make use of
`pydantic-ai` instead of `langchain` for correctness and ergonomics.
This should be a good foundation for us to build our AI engine on going
forwards.