fix(core): ignore per_item_quality for YTM playlist items

YouTube Music playlists force child_quality = Some("audio") because
yt-dlp can't download DRM-free audio-only tracks any other way. The
previous per_item_quality lookup could override this with e.g. "best",
defeating the invariant. Guard the lookup behind !is_audio so YTM items
are always downloaded as audio regardless of what the caller sends.

crates/archivr-core/src/capture.rs

@@ -1176,12 +1176,19 @@ pub fn perform_capture(
                 None => playlist_item.title.clone(),
             };
 
-            // Per-item quality override keyed by yt-dlp video ID; fall back to playlist-level.
-            let effective_child_quality: Option<&str> = config
-                .per_item_quality
-                .get(&playlist_item.id)
-                .map(String::as_str)
-                .or(child_quality);
+            // Per-item quality override keyed by yt-dlp video ID; fall back to
+            // playlist-level quality. YTM playlists are always audio-only —
+            // per_item_quality overrides are ignored so the audio-only guarantee
+            // cannot be bypassed by a caller sending e.g. "best".
+            let effective_child_quality: Option<&str> = if is_audio {
+                Some("audio")
+            } else {
+                config
+                    .per_item_quality
+                    .get(&playlist_item.id)
+                    .map(String::as_str)
+                    .or(child_quality)
+            };
 
             // Download the media.
             match downloader::ytdlp::download(