fix: make python-magic import graceful on Windows (avoids segfault when libmagic missing)

- Wrap import magic in try/except so the module can be imported on
  Windows without libmagic installed.
- Guard _identify MIME-type lookup — returns None (falling back
  to extension matching) when magic is unavailable.
- CI: skip --extras dbc and pre-commit on Windows.
- CI: per-OS timeout (30 m Windows, 15 m Linux).
- CI: guard Pages deployment steps behind refs/heads/main to avoid
  failure on PRs from forks.
- Coverage: omit pysus/management/client.py and pysus/tui/*.

Author: luabida