Implement final frame duration fallback in video_describer.py

[chigwell]

User Story
As a developer using FrameStory,
I want the final frame's duration to have a fallback value
so that video descriptions contain complete timing metadata for all frames.

Background
The describe_frames method in frame_story/video_describer.py currently leaves the last frame's duration undefined when i+1 exceeds the times array length. This creates inconsistent output formats and risks downstream processing errors for applications relying on complete duration data. The loop in describe_frames calculates durations using times[i+1] - time without handling the terminal case.

Acceptance Criteria

• Modify describe_frames in video_describer.py to implement duration fallback for final frame
• Set fallback duration using either:
  • A default value (e.g., 1 second)
  • Configurable parameter via VideoDescriber initialization
• Add test case verifying last frame's duration matches fallback strategy
• Ensure existing frame extraction logic remains unchanged
• Update method docstring to document fallback behavior
• Validate through:
  • Video with only 1 significant frame
  • Video where all frames are significant
  • Existing test suite passing