feat(controls): atom-grain path in get_controls_for_use_case

Reads compliance.atom_classification (Haiku pass: relevant + sub_topic +
canonical_obligation) when present -> precise, sub-topic-organized controls per
topic; master-grain seed stays as fallback for unprocessed topics. New optional
sub_topic filter + subtopic_counts facet + granularity flag in the response.

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

backend-compliance/compliance/api/use_case_controls_routes.py

@@ -9,7 +9,7 @@ draw from ONE controls index instead of separate retrievals. Read-only.
 
 from __future__ import annotations
 
-from typing import Any
+from typing import Any, Optional
 
 from fastapi import APIRouter, Depends, Query
 from sqlalchemy.orm import Session
@@ -39,11 +39,13 @@ async def list_use_cases(
 @router.get("/use-cases/{use_case}/controls")
 async def controls_for_use_case(
     use_case: str,
-    primary_only: bool = Query(False, description="Nur Primaerzweck-Mappings"),
+    primary_only: bool = Query(False, description="master-grain Fallback: nur Primaerzweck"),
+    sub_topic: Optional[str] = Query(None, description="atom-grain: nur dieses Sub-Thema"),
     limit: int = Query(50, ge=1, le=200),
     offset: int = Query(0, ge=0),
     svc: UseCaseControlsService = Depends(get_use_case_controls_service),
 ) -> dict[str, Any]:
-    """Controls mapped to a topic, ranked by the deterministic precision proxy."""
+    """Controls for a topic. Atom-grain (Haiku: relevant + sub_topic) wenn vorhanden,
+    sonst master-grain Seed."""
     with translate_domain_errors():
-        return svc.controls_for_use_case(use_case, primary_only, limit, offset)
+        return svc.controls_for_use_case(use_case, primary_only, limit, offset, sub_topic)