Groundwork toward PLN's "merge-of-stems multimodel": the finding that reshaped #9
is that there is NO 4-stem Roformer — all Roformer models are 2-stem; the only
4-stem (drums/bass/other/vocals) separators are the Demucs family. So the immediate,
zero-risk stem-quality A/B is between demucs variants, which the DemucsBackend already
supports via its `model` arg.

Backends now declare a `models` list (demucs: htdemucs / htdemucs_ft / hdemucs_mmi /
htdemucs_6s). /api/backends exposes it; the Foundry GUI gains a model dropdown beside
the engine picker (hidden when a backend has ≤1 model), passed through to /api/separate.
So PLN can now separate with htdemucs_ft (fine-tuned: vocals SDR 10.8 vs 9.9, bass 12.0
vs 11.6) or htdemucs_6s (adds guitar+piano stems) and compare — no new backend.

Verified: build_cmd respects the chosen model (-n htdemucs_ft), /api/backends returns
the list, the selector renders all four. The full multimodel engine2 (Roformer-vocals
+ demucs-rest merged, the door PLN asked to keep open) stays on #9 with the design.