fix(tests): skip SearXNG migration tests on Windows

[Christian-Sidak]

Summary

Guard the two POSIX-only calls in scripts/migrate_searxng_settings.py (os.fchown and os.O_DIRECTORY) behind sys.platform != "win32" checks, and add a module-level pytestmark to skip all tests in tests/test_searxng_settings_migration.py on Windows. The migration runs exclusively inside the Linux SearXNG container; the POSIX ownership and directory-fsync guarantees are preserved on all Linux paths.

Target branch

• This PR targets dev, not main. All PRs land in dev; main is curated by the maintainer at each release. If your PR is on main by accident, click "Edit" on this PR and change the base.

Linked Issue

Fixes #6067.

Type of Change

• Bug fix (non-breaking -- fixes a confirmed issue)
• New feature (non-breaking -- adds new behaviour)
• Breaking change (changes or removes existing behaviour)
• Refactor / cleanup (behaviour unchanged)
• Documentation only
• CI / tooling / configuration

Checklist

• I searched open issues and open PRs -- this is not a duplicate.
• This PR targets dev
• My changes are limited to the scope described above -- no unrelated refactors or whitespace changes mixed in.
• I actually ran the app (docker compose up or uvicorn app:app) and verified the change works end-to-end. Type-checks and unit tests are not enough.
• I did not run the app/runtime validation and stated that gap in How to Test. Leave this unchecked when the app-run box above is checked.

How to Test

1. Run python -m pytest -q tests/test_searxng_settings_migration.py on Linux/macOS -- all tests should pass as before.
2. On Windows: python -c "import importlib.util; spec = importlib.util.spec_from_file_location('m', 'scripts/migrate_searxng_settings.py'); m = importlib.util.module_from_spec(spec); spec.loader.exec_module(m)" -- should import without AttributeError.
3. Verify the pytestmark skip: python -m pytest tests/test_searxng_settings_migration.py --collect-only on Windows shows all tests as skipped with the platform reason.

The POSIX ownership (fchown) and directory-fsync (O_DIRECTORY) paths remain intact on non-Windows systems. On macOS the migration was verified to correctly insert use_default_settings: true and preserve mode 0o640 on a temp file. python -m py_compile scripts/migrate_searxng_settings.py and git diff --check pass.

Visual / UI changes -- REQUIRED if you touched anything that renders

Not applicable: no static, template, style, or rendering files changed.

• Screenshot or short clip of the change in the running app, attached below.
• Style match: the change uses Odysseus's existing visual language.
• No new component patterns.
• I am not an LLM agent submitting a bulk PR.

Screenshots / clips

N/A -- no rendering code changed.

[github-actions]

⚠️ PR description is complete; validation evidence is still outstanding

Changed-file classification: backend/runtime.

Author-reported runtime / visual state

• The author explicitly reports that app/runtime validation was not performed.

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