feat(web): support ssh owner repo clone flow

- Add SSH-only owner/repo clone path for git.commumedia.org
- Preflight remote repository existence with git ls-remote before cloning
- Keep advanced URL paste fallback and blank repository creation
- Add focused backend and frontend coverage plus docs updates

Quality gates: python -m py_compile, vitest run src/components/repositories-settings-tab.test.tsx, npm run typecheck

apps/api/tests/unit/test_git_repository_clone_preflight.py

@@ -0,0 +1,28 @@
+from unittest.mock import Mock, patch
+
+import pytest
+from fastapi import HTTPException
+
+from src.api.git_repositories import _build_provider_clone_url, _preflight_remote_repository
+
+
+def test_build_provider_clone_url_uses_fixed_host() -> None:
+    assert _build_provider_clone_url("alice", "demo") == "git@git.commumedia.org:alice/demo.git"
+
+
+def test_preflight_remote_repository_allows_accessible_repo() -> None:
+    completed = Mock(returncode=0)
+    with patch("src.api.git_repositories.subprocess.run", return_value=completed) as run_mock:
+        _preflight_remote_repository("git@git.commumedia.org:alice/demo.git")
+
+    run_mock.assert_called_once()
+
+
+def test_preflight_remote_repository_rejects_missing_repo() -> None:
+    completed = Mock(returncode=128)
+    with patch("src.api.git_repositories.subprocess.run", return_value=completed):
+        with pytest.raises(HTTPException) as exc_info:
+            _preflight_remote_repository("git@git.commumedia.org:alice/missing.git")
+
+    assert exc_info.value.status_code == 400
+    assert exc_info.value.detail == "repository not found or inaccessible"