refactor(v2): inline discogs delegates and delete legacy discogs commands

Inlines the bodies of `discogs_has_credentials`,
`discogs_search_artwork`, and `discogs_download_artwork` from
crate::library::commands::* directly into commands_v2/legacy_compat.rs.
The discogs delegates were single-line bodies (or trivial proxy logic);
DiscogsClient and library::get_artwork_cache_dir are reused as-is.

The String error type from the underlying DiscogsClient methods is
mapped to RuntimeError::Internal at the v2 boundary, matching the
existing v2 contracts.

Deletes the legacy #[tauri::command] definitions from
library/commands.rs. No frontend invoke changes required.

Step 6 of the legacy cleanup plan, group B (discogs).

cc

src-tauri/src/commands_v2/legacy_compat.rs

@@ -550,9 +550,8 @@ pub async fn v2_library_update_folder_settings(
 
 #[tauri::command]
 pub async fn v2_discogs_has_credentials() -> Result<bool, RuntimeError> {
-    crate::library::commands::discogs_has_credentials()
-        .await
-        .map_err(RuntimeError::Internal)
+    // Proxy always provides credentials.
+    Ok(true)
 }
 
 #[tauri::command]
@@ -562,7 +561,16 @@ pub async fn v2_discogs_search_artwork(
     album: String,
     catalogNumber: Option<String>,
 ) -> Result<Vec<crate::discogs::DiscogsImageOption>, RuntimeError> {
-    crate::library::commands::discogs_search_artwork(artist, album, catalogNumber)
+    log::info!(
+        "Command: v2_discogs_search_artwork {} - {} (catalog: {:?})",
+        artist,
+        album,
+        catalogNumber
+    );
+
+    let client = crate::discogs::DiscogsClient::new();
+    client
+        .search_artwork_options(&artist, &album, catalogNumber.as_deref())
         .await
         .map_err(RuntimeError::Internal)
 }
@@ -574,7 +582,13 @@ pub async fn v2_discogs_download_artwork(
     artist: String,
     album: String,
 ) -> Result<String, RuntimeError> {
-    crate::library::commands::discogs_download_artwork(imageUrl, artist, album)
+    log::info!("Command: v2_discogs_download_artwork from {}", imageUrl);
+
+    let cache_dir = crate::library::get_artwork_cache_dir();
+    let client = crate::discogs::DiscogsClient::new();
+
+    client
+        .download_artwork_from_url(&imageUrl, &cache_dir, &artist, &album)
         .await
         .map_err(RuntimeError::Internal)
 }

src-tauri/src/library/commands.rs

@@ -1868,13 +1868,6 @@ pub async fn playlist_clear_custom_order(
 
 // === Discogs Artwork ===
 
-/// Check if Discogs credentials are configured (proxy handles credentials)
-#[tauri::command]
-pub async fn discogs_has_credentials() -> Result<bool, String> {
-    // Proxy always provides credentials
-    Ok(true)
-}
-
 /// Fetch missing artwork from Discogs for albums without artwork
 /// Returns number of albums updated
 #[tauri::command]
@@ -2937,45 +2930,6 @@ pub async fn playlist_get_offline_available(
     Ok(playlists.iter().map(|p| p.qobuz_playlist_id).collect())
 }
 
-// === Discogs Artwork ===
-
-/// Search Discogs for artwork options
-#[tauri::command]
-pub async fn discogs_search_artwork(
-    artist: String,
-    album: String,
-    catalog_number: Option<String>,
-) -> Result<Vec<crate::discogs::DiscogsImageOption>, String> {
-    log::info!(
-        "Command: discogs_search_artwork {} - {} (catalog: {:?})",
-        artist,
-        album,
-        catalog_number
-    );
-
-    let client = DiscogsClient::new();
-    client
-        .search_artwork_options(&artist, &album, catalog_number.as_deref())
-        .await
-}
-
-/// Download and save Discogs artwork
-#[tauri::command]
-pub async fn discogs_download_artwork(
-    image_url: String,
-    artist: String,
-    album: String,
-) -> Result<String, String> {
-    log::info!("Command: discogs_download_artwork from {}", image_url);
-
-    let cache_dir = get_artwork_cache_dir();
-    let client = DiscogsClient::new();
-
-    client
-        .download_artwork_from_url(&image_url, &cache_dir, &artist, &album)
-        .await
-}
-
 /// Get multiple tracks by their IDs
 #[tauri::command]
 pub async fn library_get_tracks_by_ids(