Remove unused [video] OpenCV dependency

[quangshuynh]

Problem

pyproject.toml exposes an optional [video] dependency containing opencv-python, but the current codebase does not use OpenCV or provide video metadata functionality.

Keeping an unused extra makes the package surface larger than necessary and implies a capability the application does not currently implement.

Goal

Remove the unused video optional dependency so the declared package extras match the actual supported functionality.

Acceptance criteria

• The video optional dependency is removed from pyproject.toml.
• No OpenCV dependency remains in the project metadata.
• Existing image/HEIF metadata behavior remains unchanged.
• Existing tests and lint checks continue to pass.
• Documentation is updated only if it currently references installing or using the video extra.

Non-goals

• Adding video metadata extraction.
• Introducing a replacement video library.
• Changing image or HEIF metadata handling.
• Refactoring unrelated dependency declarations.

Relevant files

• pyproject.toml
• documentation only if the video extra is referenced

Contribution notes

Please keep this as a removal-only cleanup. Do not add video support as part of this issue.