Wires IGachaPointService.TryExchangeAsync into a controller endpoint.
Loads the viewer with the full cosmetic-grant graph (Cards, Sleeves,
Emblems, Degrees, LeaderSkins, MyPageBackgrounds) plus the gacha-point
balance + received marker, with AsSplitQuery to avoid the cartesian
explosion documented in project_ef_split_query. Returns BadRequest with
the outcome's error code on validation failure, or the post-state
reward_list on success.
Two integration tests: happy-path verifies card grant + balance debit +
received-marker persistence + post-state reward_list shape; insufficient-
balance path returns 400.
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
