Add photo station capture and reuse workflow - #137

Summary

This adds the full reusable photo-station workflow to tracing. Users can capture from a camera, crop the capture area, reuse a saved station's corners when the setup matches, save the current setup as a new station, and continue into a save-and-new loop for repeated captures.

The PR includes:

• a /trace camera capture page
• capture crop overlay controls
• station-aware upload metadata
• station suggestions in the trace corner step
• one-click reuse of matching station corners
• corner redetection from the trace page
• save-as-station support while confirming corners
• save-and-new flow for repeated station captures
• frontend API tests for station request payloads and capture upload metadata

Dependency

This PR depends on #135 and #136.

Because these branches live in a fork and the base repository does not have the stacked base branches, this PR targets main. Until the earlier PRs land, GitHub's diff may include commits from those PRs as part of the stacked branch history.

Implementation Notes

This PR depends on the backend APIs and management UI from the previous two PRs.

The capture page can receive a selected station and pass that station id through upload. When the backend can apply the saved station during upload, the trace editor reflects that reused station state. If the station does not match the current image, the UI disables non-matching station options and reports a warning rather than silently applying bad corners.

The trace editor station logic was extracted into a dedicated usePhotoStations hook. That keeps station suggestions, reuse, redetect, active station state, save-as-station state, and station notices out of the already-large trace page component.

This also addresses review feedback from the original larger PR:

• Wraps startCamera in useCallback.
• Removes duplicated station-state logic from the trace page.
• Adds frontend test coverage for the API request shapes used by the station workflow.

Tests

• corepack pnpm@10.29.2 --dir frontend exec vitest run
• corepack pnpm@10.29.2 --dir frontend lint
• corepack pnpm@10.29.2 --dir frontend exec tsc --noEmit
• python -m pytest backend/tests/test_photo_stations.py

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