Hide MLX-only variants on non-Apple platforms

[cryptopoly]

Summary

• Adds backend_service/helpers/platform_filter.py with is_apple_silicon(), is_mlx_only_variant(), and filter_mlx_only_families().
• Wires the filter into _image_model_payloads and _video_model_payloads so the /api/images/catalog and /api/video/catalog responses drop MLX-only variants on Linux / Windows.
• Drops MLX-only families entirely when their full variant set is Apple-only (e.g. the LTX-2 (MLX) family disappears on Windows rather than rendering an empty card).
• Covers the helper with 15 unit tests in tests/test_platform_filter.py — pinned platform values keep the suite reliable on any host.

Why

Surfaced by the v0.7.2 smoke test on a Windows / RTX 4090 box. The Image Studio dropdown showed FLUX.1 Dev · mflux (MLX) alongside the GGUF variants; Video Studio showed the four prince-canuma/LTX-2-* MLX entries. Both routes depend on mlx, which has no Linux or Windows wheels — picking either failed at preload time.

Filtering is server-side so no frontend code changes are needed and the dropdowns stay honest with whatever the catalog ships.

Detection

A variant is treated as MLX-only iff one of the following is true:

• mlxOnly: True (explicit override for future entries)
• engine ∈ {mflux, mlx-video}
• runtime contains mflux (MLX native) or mlx-video (MLX native)

The detection is conservative: false negatives leave a broken dropdown entry that the smoke test will catch; false positives would silently hide a working variant.

Test plan

• .venv/bin/python -m pytest tests/test_platform_filter.py -v — 15 / 15 pass
• .venv/bin/python -m pytest tests/test_backend_service.py tests/test_setup_routes.py tests/test_video_routes.py tests/test_image_discover.py -q — all pre-existing tests still pass
• Manual verify on macOS: MLX entries still appear in Image Studio + Video Studio
• Manual verify on Windows: MLX entries no longer appear; GGUF / diffusers entries unchanged