Resolve in-pack audio on backend

Move audio member selection logic from frontend to backend. The frontend was probing multiple candidate audio files with error/retry logic, often resulting in 404s. Now the backend resolves the best available audio file (preview.ogg, stems/full.ogg, or any stem) during metadata extraction and includes it in song metadata as `audio_member`. The frontend makes a single, guaranteed request instead of multiple probes. Simplifies the preview flow and improves reliability.

Signed-off-by: Kyle <kyle.j.t@live.co.uk>

plugins/folder_library/routes.py

@@ -11,6 +11,7 @@ from fastapi import APIRouter, Request
 from fastapi.responses import JSONResponse
 import shutil
 import re
+import zipfile
 
 
 # ── Pure, testable helpers ─────────────────────────────────────────────────
@@ -114,6 +115,37 @@ def setup(app, context):
         sloppak = dlc / "sloppak"
         return sloppak if sloppak.exists() else dlc
 
+    def _audio_member(p: Path):
+        """Best in-pack audio file for hover-preview, as a pack-relative path.
+
+        Prefers a dedicated preview clip, then a full mix, then any stem — so the
+        frontend can preview with a single request instead of probing (and 404ing)
+        a hardcoded path. Returns None when the pack carries no audio.
+        """
+        prefer = ("preview.ogg", "stems/full.ogg", "stems/audio.mp3",
+                  "stems/audio.ogg", "audio.mp3", "audio.ogg", "full.ogg")
+        audio_ext = (".ogg", ".mp3", ".opus", ".m4a", ".oga")
+        try:
+            if p.is_dir():
+                names = ["/".join(f.relative_to(p).parts) for f in p.rglob("*") if f.is_file()]
+            else:
+                with zipfile.ZipFile(p) as z:
+                    names = z.namelist()
+        except Exception:
+            return None
+        nameset = set(names)
+        for m in prefer:
+            if m in nameset:
+                return m
+        for n in names:                    # any stem audio
+            low = n.lower()
+            if low.startswith("stems/") and low.endswith(audio_ext):
+                return n
+        for n in names:                    # any audio at all
+            if n.lower().endswith(audio_ext):
+                return n
+        return None
+
     def _meta(p: Path, dlc: Path) -> dict:
         # filename and added are always computed fresh — they change when files move.
         try:
@@ -136,7 +168,8 @@ def setup(app, context):
 
         # Cache miss — run the expensive extract.
         m = {"title": None, "artist": None, "album": None, "duration": None,
-             "year": None, "tuning": None, "arrangements": [], "stems": [], "lyrics": False}
+             "year": None, "tuning": None, "arrangements": [], "stems": [], "lyrics": False,
+             "audio_member": _audio_member(p)}
         try:
             raw = context["extract_meta"](p)
             if raw:

plugins/folder_library/screen.js

@@ -743,11 +743,13 @@ function createFolderSurface(cfg) {
     var _HOVER_PREVIEW_DELAY_MS = 500;
     var _previewIndHost = null;             // art element currently showing the indicator
 
-    // Pack audio layout varies: a dedicated preview.ogg (most songs), a single
-    // stems/full.ogg mix, or tutorials' stems/audio.mp3 — try in that order.
-    function _previewCandidates(song) {
-        var base = '/api/sloppak/' + song.filename.split('/').map(encodeURIComponent).join('/') + '/file/';
-        return [base + 'preview.ogg', base + 'stems/full.ogg', base + 'stems/audio.mp3'];
+    // The backend resolves the correct in-pack audio member (song.audio_member),
+    // so we preview with a single request — no probing / 404s. Null = no audio.
+    function _previewUrl(song) {
+        if (!song || !song.audio_member) return null;
+        var enc = song.filename.split('/').map(encodeURIComponent).join('/');
+        var mem = song.audio_member.split('/').map(encodeURIComponent).join('/');
+        return '/api/sloppak/' + enc + '/file/' + mem;
     }
 
     function _ensurePreviewStyle() {
@@ -808,24 +810,16 @@ function createFolderSurface(cfg) {
         // full-track fallbacks are fine from the start. (Seeking by a fraction
         // of song.duration broke playback whenever the offset landed past the
         // end of a short preview clip.)
-        var urls = _previewCandidates(song);
-        var a    = _previewEl();
-        var seq  = ++_previewSeq;
-        var idx  = 0;
+        var url = _previewUrl(song);
+        if (!url) return;                    // pack carries no previewable audio
+        var a   = _previewEl();
+        var seq = ++_previewSeq;
         _showIndicator(host);
-        function _tryNext() {
-            if (seq !== _previewSeq) return;
-            if (idx >= urls.length) { _clearIndicator(); return; }  // no playable member
-            a.src = urls[idx++];
-            a.load();
-        }
-        a.onerror = function () { if (seq === _previewSeq) _tryNext(); };   // candidate 404/decode → next
-        a.onloadedmetadata = function () {
-            if (seq !== _previewSeq) return;
-            a.play().catch(function () {});
-        };
-        a.onplaying = function () { if (seq === _previewSeq) _markIndicatorPlaying(); };
-        _tryNext();
+        a.onerror         = function () { if (seq === _previewSeq) _clearIndicator(); };
+        a.onloadedmetadata = function () { if (seq === _previewSeq) a.play().catch(function () {}); };
+        a.onplaying       = function () { if (seq === _previewSeq) _markIndicatorPlaying(); };
+        a.src = url;
+        a.load();
     }
     function _armHoverPreview(el, song, host) {
         el.addEventListener('mouseenter', function () {