security: require auth for uploaded photos (GHSA-wxx3-84fc-mrx2)
GHSA-pcr3-6647-jh72 (HIGH):
- Add canAccessTrip check to all /trips/:tripId/photos and
/trips/:tripId/album-links endpoints
- Prevents authenticated users from accessing other trips' photos
GHSA-wxx3-84fc-mrx2 (LOW):
- /uploads/photos now requires JWT auth token or valid share token
- Covers and avatars remain public (needed for login/share pages)
- Files were already blocked behind auth
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