fix: get_tree returns 404 (not 403) for inaccessible trees — don't leak resource existence

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

backend/app/api/endpoints/trees.py

@@ -392,9 +392,10 @@ async def get_tree(
         )
 
     if not tree.is_active or not can_access_tree(current_user, tree):
+        # Always 404, never 403. A 403 confirms the resource exists.
         raise HTTPException(
-            status_code=status.HTTP_403_FORBIDDEN,
-            detail="You don't have access to this tree"
+            status_code=status.HTTP_404_NOT_FOUND,
+            detail="Tree not found"
         )
 
     return build_full_tree_response(tree)

backend/tests/test_trees.py

@@ -447,3 +447,55 @@ class TestVisibilityFilter:
         assert "author_name" in trees[0]
         # visibility key should be present
         assert "visibility" in trees[0]
+
+    @pytest.mark.asyncio
+    async def test_get_tree_returns_404_not_403_for_other_account_tree(
+        self, client: AsyncClient, auth_headers: dict, test_db: AsyncSession
+    ):
+        """Account A must not learn that Account B's private tree exists."""
+        from app.models.tree import Tree
+        from app.models.account import Account
+        from app.models.user import User
+        from app.core.security import get_password_hash
+        import uuid
+
+        # Create a second account and user
+        account_b = Account(name="Other Corp", display_code="OTH00001")
+        test_db.add(account_b)
+        await test_db.flush()
+
+        user_b = User(
+            email=f"user-b-{uuid.uuid4().hex[:6]}@example.com",
+            name="User B",
+            password_hash=get_password_hash("TestPass123!"),
+            is_active=True,
+            account_id=account_b.id,
+            account_role="engineer",
+        )
+        test_db.add(user_b)
+        await test_db.flush()
+
+        # Create a private tree belonging to account_b
+        private_tree = Tree(
+            name="Secret Tree",
+            account_id=account_b.id,
+            author_id=user_b.id,
+            visibility="private",
+            tree_type="troubleshooting",
+            tree_structure={"id": "root", "type": "start", "children": []},
+            is_active=True,
+            is_default=False,
+            is_public=False,
+            status="published",
+        )
+        test_db.add(private_tree)
+        await test_db.commit()
+
+        response = await client.get(
+            f"/api/v1/trees/{private_tree.id}",
+            headers=auth_headers,
+        )
+        assert response.status_code == 404, (
+            f"Expected 404 but got {response.status_code} — "
+            "leaking tree existence to wrong tenant"
+        )