Improve player backend diagnostics

[pitill0]

Summary

Improve player backend diagnostics and align Flatpak/AppStream metadata with the new backend capabilities model.

Changes

• Extend fluxtuner --list-players to show backend capability details.
• Show whether each backend is general-purpose or specialized.
• Show declared codecs for specialized backends.
• Show supported live controls when available.
• Update Flatpak documentation so it no longer only mentions mpv and ffplay.
• Clarify that the local Flatpak manifest does not currently bundle player binaries explicitly.
• Update AppStream metadata to describe backend detection generically instead of listing specific backends.
• Add tests for the player capability summary helper.

Validation

• python -m fluxtuner --list-players
• ruff check .
• python -m pytest
• python -m bandit -r fluxtuner
• python -m pip_audit