fix(library): extract track/disc number from file tags during backfill import

insert_qobuz_cached_track_direct was hardcoding track_number to 0,
causing all imported tracks to show wrong numbers. Now accepts
track_number and disc_number params, and both callers (backfill
downloads and offline cache sync) extract metadata from the audio
file via MetadataExtractor before inserting.

Also fix LocalLibraryView to use || instead of ?? for track_number
fallback, so 0 correctly falls back to index-based numbering.

Closes #147

crates/qbz-library/src/database.rs

@@ -3348,6 +3348,8 @@ impl LibraryDatabase {
         file_path: &str,
         bit_depth: Option<u32>,
         sample_rate: Option<f64>,
+        track_number: Option<u32>,
+        disc_number: Option<u32>,
     ) -> Result<(), LibraryError> {
         use std::time::SystemTime;
 
@@ -3378,8 +3380,8 @@ impl LibraryDatabase {
                 artist,
                 album.unwrap_or("Unknown Album"),
                 artist, // Use artist as album_artist for proper grouping
-                0, // track_number - will be updated if metadata is available
-                None::<u32>, // disc_number
+                track_number.map(|v| v as i64),
+                disc_number.map(|v| v as i64),
                 None::<u32>, // year
                 duration_secs as i64,
                 "flac", // Default format for downloads