Apply backend compatibility to legacy CLI

[pitill0]

Summary

Apply backend compatibility checks to the legacy numbered CLI so it behaves consistently with the Textual TUI and GTK GUI when specialized player backends are selected.

Changes

• Add helper functions for backend capabilities and CLI station compatibility.
• Filter search results for specialized backends.
• Filter favorites and random favorite playback for specialized backends.
• Block incompatible playback attempts with the existing unsupported-backend message.
• Add tests for CLI compatibility filtering.
• Add a short usage note for legacy CLI behavior.

Validation

• ruff check .
• python -m pytest tests/test_cli_compatibility.py
• python -m pytest
• python -m bandit -r fluxtuner
• pip-audit --local