Add race and station management

- races table: name, date, description, is_active
- stations table: ordered checkpoints with GPS per race
- New /api/races and /api/races/{id}/stations endpoints
- Upload now requires race + station selection; uses station GPS
  so images without GPS EXIF are accepted
- passages filtered by active race throughout
- RacePage: create races, manage stations (add/edit/delete checkpoints)
- Navbar shows active race name
- Start and finish stations created automatically per race

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

backend/requirements.txt

@@ -2,7 +2,7 @@ fastapi==0.115.6
 uvicorn[standard]==0.34.0
 pillow==11.1.0
 piexif==1.1.3
-easyocr==1.7.2
+pytesseract==0.3.13
 opencv-python-headless==4.11.0.86
 python-multipart==0.0.20
 watchdog==6.0.0