Add photo station backend - #135

Summary

This adds the backend foundation for reusable photo stations. A photo station stores a known camera/paper setup so later captures from the same setup can reuse or compare against saved paper corners instead of starting from scratch each time.

The PR includes:

• photo station request/response schemas
• a JSON-backed photo station store
• endpoints for listing, creating, updating, deleting, and reusing stations
• station suggestions for an uploaded session
• corner redetection support
• saving a station directly while confirming corners
• focused backend tests for the new station behavior

Implementation Notes

The station store follows the existing local JSON-store pattern used elsewhere in the backend. Station records include paper size, image dimensions, source image path, saved corners, and timestamps.

The corner confirmation endpoint can now optionally receive save_station_name. When present, the backend creates a station from the confirmed session image and corners as part of the same workflow.

This also addresses review feedback from the original larger PR:

• Uses timezone-aware UTC timestamps instead of datetime.utcnow().
• Centralizes station creation logic in a helper instead of duplicating it between routes.
• Copies the original uploaded image into station storage before unlinking the upload source.
• Adds backend coverage for station creation, suggestion behavior, invalid input, corrupt store handling, and the copy-before-unlink path.

Tests

• python -m pytest backend/tests/test_photo_stations.py

Code and PR drafted with Codex