Get Layer Mvt Agg Tile

Serve an aggregate (bin-count) MVT tile for low-zoom rendering.

Emits one MVT point per occupied grid cell with a count property — the client renders bubbles sized by count at zoom levels below the detail-layer threshold so panning over dense data doesn't tessellate tens of thousands of features the user can't actually see.

filter_id — same as detail tiles; the aggregate counts then reflect the active filter instead of the layer total. Favorites-only resolution matches get_layer_mvt_tile so agg bubbles count only the favorites when that toggle is on.