Store task `location` / `point` on LineString geometry (not only geometric center)

What would you like to see?

Summary

For tasks whose geometry is a LineString / MultiLineString, MapRoulette’s stored location (API point from /tasks/box) is often a geometric center that does not lie on the line. On curved ways the marker sits beside the road.

maproulette3 already works around this in the frontend by snapping markers to the nearest point on the line (PR maproulette/maproulette3#1041 — nearestPointToCenter() via @turf/nearest-point-on-line in AsMappableTask). Other clients (e.g. iD, Rapid) that place pins from task.point alone still show off-line markers.

Why is this useful?

Ask

Please compute and persist a display location that lies on the line geometry in the backend (or when tasks are created/updated), so tasks.location / API point is correct for all consumers—not only maproulette3’s map UI.

Suggested approach (same idea as the frontend):

1. Take the current center / centroid of the task geometries
2. Project it onto the line(s) with something like PostGIS ST_ClosestPoint (or equivalent)
3. Store that as location

Why backend?

• One fix for every editor/API consumer
• Avoids requiring includeGeometries / per-client Turf logic just to place a pin
• Matches what maproulette3 already does for display https://github.com/maproulette/maproulette3/pull/1041

Edge cases to decide

• Mixed FeatureCollections (Point + LineString): which geometry to snap to?
• Multi-feature / MultiLineString tasks
• Migration / rebuild for existing tasks

Additional context

No response

Trace task creation and update paths in the Scala backend to find where task geometries and the persisted location/API point are calculated. Review the proposed PostGIS closest-point approach, then define behavior for LineString, MultiLineString, mixed features, and existing-task migration; done means stored points lie on the relevant line geometry for all consumers.