fix: preserve file extension on image rename; add reverse-side review for bi cards
- ImageTile rename now uses the original file extension instead of hardcoding .png
- tala-cli review: bi-directional cards enqueue a "reverse" item (back->front)
in addition to "forward"; apply_schedule and print_card_preview handle sub_id="reverse"
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>