Make DeepLabCut an optional dependency to fix circular dependency issue

[deruyter92]

Description:
Make deeplabcut an optional dependency in fmpose3d to break the new circular dependency chain (deeplabcut[fmpose3d] -> fmpose3d -> deeplabcut).
This PR removes deeplabcut from core install requirements, adds it as an animals extra, updates DLC import paths to fail gracefully with clear install guidance when missing, and updates docs/tests accordingly.

Changes included:

• Removed deeplabcut from base required dependencies in pyproject.toml.
• Added a new optional extra for animal workflows: fmpose3d[animals] installs deeplabcut.
• Unpin deeplabcut version -> make it a lower bound.
• Updated animal-related DLC imports to be guarded and fail with a clear, actionable error message when DLC is not installed (including install hint).
• Kept the human pipeline unaffected when DLC is absent.
• Updated docs (README.md and fmpose3d/inference_api/README.md) to document optional installation for animal inference.
• Updated tests/skip logic to handle environments without DLC more cleanly and verify the new missing-dependency error path.

This preserves existing animal functionality when the extra is installed, while removing the hard dependency edge that caused the cycle.

[deruyter92]

@xiu-cs, do you think we can replace the pinned DeepLabCut version to a more flexible one, e.g. only a lowerbound?
Might be more future-proof than pinning the required version to this specific pre-release

[xiu-cs]

@xiu-cs, do you think we can replace the pinned DeepLabCut version to a more flexible one, e.g. only a lowerbound? Might be more future-proof than pinning the required version to this specific pre-release

Thanks! Previous I only tested the rc14 release version; other versions may also work.