Pass per-stem name/description through to the stems payloads (#1013)

feedpak 1.16.0 (spec §5.3) added two OPTIONAL presentational fields to a
stems[] entry: `name` (display label, Readers fall back to the id) and
`description` (free text). The server dropped both while normalizing
manifest stems, so no client could ever display them.

Pass them through at the one place stem descriptors are built
(sloppak.load_song) and let both payload builders — the WS `ready` stems
list and the REST `/api/song/{f}?stems=1` preload list, which are pinned
against each other by test — carry them forward. Omit-when-absent, so a
stem without the fields does not grow null keys; non-string or blank
values are dropped rather than surfaced.

No behaviour change for existing packs or clients: the fields are
additive and every consumer that reads {id,url,default} keeps working
unchanged. The stems plugin / stem mixer display work lands separately.

Signed-off-by: topkoa <topkoa@gmail.com>
Co-authored-by: Claude Fable 5 <noreply@anthropic.com>

lib/routers/song.py

@@ -868,7 +868,8 @@ def _playable_stems_payload(filename: str, dlc) -> dict:
 
     return {
         "stems": [
-            {"id": s["id"], "url": _url(s["file"]), "default": s["default"]}
+            {"id": s["id"], "url": _url(s["file"]), "default": s["default"],
+             **{k: s[k] for k in ("name", "description") if k in s}}
             for s in loaded.stems
         ],
         "full_mix_url": _url(loaded.full_mix) if loaded.full_mix else None,