refactor(server): decompose the decisions router's long handlers (#2602)

* refactor(server): decompose the decisions router's long handlers

list_decisions hands the lane page cut and the evidence previews to
_page_of_lane and _attach_evidence. The settings PUT applies the body
through _apply_settings_update, with one ValueError-to-400 guard in
place of three copies. The four handlers that look a decision up share
_decision_in_repo, create_decision's scope-withdrawal check becomes
_is_accepted_with_scope, and patch_decision's status move becomes
_transition_status.

* docs(server): shorten comments in the decisions router