fix: prepare SSH keys with container UID/GID on host before mounting

- Extend prepare_ssh_key_files() with optional uid/gid parameters
- Call os.chown on created files when uid/gid are provided
- Gracefully handle PermissionError if API process is not root
- In start_instance, extract container user UID/GID from manifest
- Pass container UID/GID when preparing instance-level SSH key mounts
- Legacy clone-mode SSH keys continue to use root (0,0)
- Add unit tests for prepare_ssh_key_files ownership logic
- Keep apply_ssh_permissions() as fallback for cases where host chown fails

Quality gates: pytest 239 passed (6 pre-existing failures), tsc --noEmit clean

apps/api/tests/unit/test_ssh_keys.py

@@ -0,0 +1,63 @@
+"""Unit tests for SSH key preparation."""
+
+import os
+from pathlib import Path
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from src.services.ssh_keys import prepare_ssh_key_files
+
+
+class TestPrepareSshKeyFiles:
+    """Tests for prepare_ssh_key_files."""
+
+    @patch("src.services.ssh_keys._get_fernet")
+    def test_creates_files_with_default_permissions(self, mock_fernet, tmp_path) -> None:
+        mock_fernet.return_value.decrypt.return_value = b"private-key-content"
+        ssh_key = MagicMock()
+        ssh_key.private_key_encrypted = "enc"
+        ssh_key.public_key = "ssh-ed25519 AAA test@test"
+
+        ssh_dir = prepare_ssh_key_files(str(tmp_path), ssh_key)
+
+        assert Path(ssh_dir).exists()
+        assert (Path(ssh_dir) / "id_ed25519").exists()
+        assert (Path(ssh_dir) / "id_ed25519.pub").exists()
+        assert (Path(ssh_dir) / "config").exists()
+        assert oct(os.stat(Path(ssh_dir) / "id_ed25519").st_mode)[-3:] == "600"
+
+    @patch("src.services.ssh_keys._get_fernet")
+    def test_sets_ownership_when_uid_gid_provided(self, mock_fernet, tmp_path) -> None:
+        mock_fernet.return_value.decrypt.return_value = b"private-key-content"
+        ssh_key = MagicMock()
+        ssh_key.private_key_encrypted = "enc"
+        ssh_key.public_key = "ssh-ed25519 AAA test@test"
+
+        with patch("os.chown") as mock_chown:
+            ssh_dir = prepare_ssh_key_files(
+                str(tmp_path), ssh_key, uid=1001, gid=1001
+            )
+
+        # os.chown is called for the directory and each of the 3 files
+        assert mock_chown.call_count == 4
+        # First call is the directory
+        assert mock_chown.call_args_list[0][0][1] == 1001
+        assert mock_chown.call_args_list[0][0][2] == 1001
+
+    @patch("src.services.ssh_keys._get_fernet")
+    def test_gracefully_handles_permission_error_on_chown(
+        self, mock_fernet, tmp_path
+    ) -> None:
+        mock_fernet.return_value.decrypt.return_value = b"private-key-content"
+        ssh_key = MagicMock()
+        ssh_key.private_key_encrypted = "enc"
+        ssh_key.public_key = "ssh-ed25519 AAA test@test"
+
+        with patch("os.chown", side_effect=PermissionError("not allowed")):
+            # Should not raise
+            ssh_dir = prepare_ssh_key_files(
+                str(tmp_path), ssh_key, uid=1001, gid=1001
+            )
+
+        assert Path(ssh_dir).exists()