Support CUDA 12 container builds

[pzelasko]

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What does this PR do ?

• support CUDA 12

# This CI Dockerfile supports CUDA 13 and CUDA 12 from a single BASE_IMAGE
# build arg. The default is the recommended CUDA 13 image:
#   nvcr.io/nvidia/cuda-dl-base:26.04-cuda13.2-devel-ubuntu24.04
# The current recommended CUDA 12 image is:
#   nvcr.io/nvidia/cuda-dl-base:25.06-cuda12.9-devel-ubuntu24.04
#
# The build derives CUDA_FLAVOR internally from BASE_IMAGE by matching
# "cuda13" or "cuda12" in the image tag. That flavor selects the matching uv
# extra (cu13 or cu12) and CUDA Python package include path. If BASE_IMAGE does
# not contain either token, the build fails early.
#
# Example CUDA 13 H100+ build:
#   docker buildx build -f docker/Dockerfile.ci \
#     --build-arg GPU_TARGET=h100plus .
# Example CUDA 12 A100 build:
#   docker buildx build -f docker/Dockerfile.ci \
#     --build-arg BASE_IMAGE=nvcr.io/nvidia/cuda-dl-base:25.06-cuda12.9-devel-ubuntu24.04 \
#     --build-arg GPU_TARGET=a100 .

• remove Dockerfile.stable
• fix aistore dependency so it installs correctly, add smart_open too
• add optional ffmpeg installation (disabled by default) and torchcodec, in container only (not NeMo depending)

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[pzelasko]

/ok to test 15815f8