Compartimentation : isolation stricte des données par espace de travail

- Ajout clause else IS NULL sur tous les endpoints list (protocols, decisions,
  mandates, documents, groups, votes) — sans X-Organization → données globales
  seulement, jamais le contenu d'un autre espace
- _get_protocol/_get_decision/_get_mandate : org_id propagé à tous les
  endpoints GET/PUT/DELETE/advance/assign/revoke/steps → 404 si UUID d'un
  autre espace
- votes.py : list_vote_sessions filtre via JOIN VotingProtocol.organization_id
- groups.py : suppression _org_id_from_header() mort, create_group assigne
  organization_id correctement

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

backend/app/routers/votes.py

@@ -25,6 +25,7 @@ from app.schemas.vote import (
     VoteSessionListOut,
     VoteSessionOut,
 )
+from app.dependencies.org import get_active_org_id
 from app.services.auth_service import get_current_identity
 from app.services.vote_service import (
     close_session as svc_close_session,
@@ -170,14 +171,22 @@ def _compute_result(
 @router.get("/sessions", response_model=list[VoteSessionListOut])
 async def list_vote_sessions(
     db: AsyncSession = Depends(get_db),
+    org_id: uuid.UUID | None = Depends(get_active_org_id),
     session_status: str | None = Query(default=None, alias="status", description="Filtrer par statut (open, closed, tallied)"),
     decision_id: uuid.UUID | None = Query(default=None, description="Filtrer par decision_id"),
     skip: int = Query(default=0, ge=0),
     limit: int = Query(default=50, ge=1, le=200),
 ) -> list[VoteSessionListOut]:
     """List all vote sessions with optional filters by status and decision_id."""
-    stmt = select(VoteSession)
+    stmt = (
+        select(VoteSession)
+        .join(VotingProtocol, VoteSession.voting_protocol_id == VotingProtocol.id)
+    )
 
+    if org_id is not None:
+        stmt = stmt.where(VotingProtocol.organization_id == org_id)
+    else:
+        stmt = stmt.where(VotingProtocol.organization_id.is_(None))
     if session_status is not None:
         stmt = stmt.where(VoteSession.status == session_status)
     if decision_id is not None: