Fix elevation vs elevation_gain field-name mismatch

Problem

The DB column + Activity model use elevation, but the store/UI and leaderboard read elevation_gain (e.g. leaderboard.stx totalElevation sums a.elevation_gain). So elevation never flows through.

Acceptance

• Standardize on one name across model, ActivityStore/Index/Show responses, store, and views.

Done: standardized on elevation through the activity index/show responses (elevationGain) and store mapping. Commit c9007b4.