changes requested

blockers

PR scope -- at +2926 lines across 28 files, this bundles at least four independently shippable concerns: (1) photo station CRUD backend, (2) capture page + camera workflow, (3) save-and-new loop + skip-to-save, (4) misc polish (sidebar width, bin preview bg, popover surfaces, polygon sizing). splitting these would make review safer and revert cleaner.

fragile file deletion ordering -- in set_corners, orig_path and orig both resolve session.original_image_path via _abs(). copy happens from orig_path, then orig deletes the same file. works by accident of ordering. use a single variable or copy after the unlink guard.

should fix

• datetime.utcnow() is deprecated since Python 3.12. new code should use datetime.now(timezone.utc)
• startCamera in trace page is not wrapped in useCallback and missing from useEffect dependency array -- React rules-of-hooks violation
• trace page now has 16 useState hooks with interleaved station state. the station logic should be extracted to a usePhotoStation hook or dedicated component
• duplicated station creation logic between POST /photo-stations endpoint and inline in set_corners -- extract a shared helper
• no test for LIST /api/photo-stations, no invalid input tests (wrong corner count, missing corners), no frontend tests

nits

• sidebar width change (200px to 300px) is unrelated -- separate PR
• magic tolerance 0.001/0.999 in _is_full_capture_crop should be named constants
• _load silently swallows all exceptions with except Exception: self._stations = {} -- corrupt JSON silently loses all stations

good feature overall -- the station reuse concept is solid and the backend test coverage for the core workflow is thorough. the main ask is splitting this into smaller PRs.