feat(controls): shared get_controls_for_use_case retrieval API

Read-only layer (service + thin route + tests) that returns the controls
mapped to a use-case/topic, ranked by a deterministic precision proxy
(is_primary + mapping confidence + registry keyword relevance) over the
existing mc_use_case_mappings seed. No schema change.

Shared handoff point: the document specialist agents AND the CRA
finding-mapper draw from this one controls index instead of separate
retrievals.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

backend-compliance/compliance/api/__init__.py

@@ -58,6 +58,7 @@ _ROUTER_MODULES = [
     "canonical_control_routes",
     "control_generator_routes",
     "crosswalk_routes",
+    "use_case_controls_routes",
     "process_task_routes",
     "evidence_check_routes",
     "vvt_library_routes",