Add deduplication: keep image closest to camera within 2s window

Within a burst sequence from the same station, the image where the athlete
is physically closest to the camera gives the most accurate passage timestamp.
Proximity is measured by bib bounding box area (larger = closer).

When a duplicate is detected:
- New image closer: update timestamp + image path, delete old image
- Existing image closer: discard new image

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/ingest.py

@@ -113,6 +113,7 @@ async def process_image(path: Path) -> None:
             gps_lon=meta.gps_lon,
             gps_alt=meta.gps_alt,
             confidence=confidence,
+            proximity_score=ocr.proximity_score,
             id_method=id_method,
             source_image=str(dest),
             needs_review=needs_review,