Add function to extract evenly spaced frames from a video

[GauravSRC]

Description

Adds extract_frames_uniform to ethology.io.video -- a function
that extracts a user-specified number of evenly spaced frames from a
video file, optionally restricted to a time interval
[start_time, end_time].

What is new

• ethology/io/video.py -- new module containing
  extract_frames_uniform. Timestamps are computed with
  numpy.linspace over the requested interval; each timestamp is
  converted to the nearest frame index and sought with
  cv2.CAP_PROP_POS_FRAMES for codec-agnostic reliability.
  Output frames are saved as JPEG files named
  frame_NNNN_tT.TTTs.jpg.
• ethology/io/__init__.py -- exports extract_frames_uniform.
• tests/fixtures/video.py -- fixture that builds a 30-frame
  synthetic AVI video (10 fps, 3 s, 64×64 px) in tmp_path for
  use in tests.
• tests/test_unit/test_io/test_video.py -- unit tests covering
  correct frame counts, file existence, filename format, automatic
  output-directory creation, time-interval extraction, and all
  invalid-input error paths.
• opencv-python added to project dependencies.

How to verify

pytest tests/test_unit/test_io/test_video.py -v

Checklist

• New feature
• References issue Extract frames to label uniformly #6
• Tests added for all new functionality
• Docstrings follow numpydoc style
• Pre-commit hooks pass