fix(config): keep a Hugging Face model ID out of path normalisation

[fg618455-droid]

Summary

• whisper_model accepts a size name (`medium`), a Hugging Face repo ID (`owner/model`), or a local directory, but every value went through path expansion.
• On Windows, path expansion rewrote the separator in a repo ID like deepdml/faster-whisper-large-v3-turbo-ct2 to a backslash, and the loader then rejected the result as an invalid model size, so no model outside the built-in size names could be used on that platform.
• Expansion now only applies to what is recognisably a path (a tilde, an absolute path, or something that exists on disk); everything else is treated as an identifier and passed through unchanged.

Test plan

• pytest tests/test_config_path_expansion.py (7 tests, new coverage for the size/repo-ID/path distinction)
• Verified against the real model: deepdml/faster-whisper-large-v3-turbo-ct2 loads correctly with HF_HUB_OFFLINE=1 on Windows after this fix

[fg618455-droid]

Self-review (APPROVE not permitted on own PR via API): clean, well-scoped fix for a real Windows-only bug (path normalisation mangling a Hugging Face repo ID's separator). Minimal diff, good test coverage (repo-ID pass-through + still-normalises-a-real-directory), verified against the real model end-to-end per the PR description. No blocking issues; CI green.