fix: scope category tree_count to requesting account

tree_count on GET /categories/{id} was including trees from all
accounts, leaking cross-tenant row counts.

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

backend/app/api/endpoints/categories.py

@@ -12,6 +12,7 @@ from app.models.user import User
 from app.schemas.category import CategoryCreate, CategoryUpdate, CategoryResponse, CategoryListResponse
 from app.api.deps import get_current_active_user
 from app.core.permissions import can_manage_category, can_create_category
+from app.core.filters import tenant_filter
 
 router = APIRouter(prefix="/categories", tags=["categories"])
 
@@ -108,10 +109,12 @@ async def get_category(
             detail="You don't have access to this category"
         )
 
-    # Get tree count
+    # Get tree count — scoped to the requesting account so cross-account
+    # trees in shared categories are not counted.
     count_query = select(func.count(Tree.id)).where(
         Tree.category_id == category.id,
-        Tree.is_active == True
+        Tree.is_active == True,
+        tenant_filter(Tree, current_user.account_id),
     )
     count_result = await db.execute(count_query)
     tree_count = count_result.scalar() or 0