perf: optimize cross-library move detection for single-library setups (#4888)

* feat: skip cross-library detection for single library setup

When only one library is configured, skip the cross-library move detection stage entirely as there are no other libraries to search in. This eliminates unnecessary database queries - the primary performance issue reported by users (5-6 hour scans with 13.5k missing files).

Implementation:
- Added library count check in processCrossLibraryMoves
- Returns input unchanged when len(state.libraries) == 1
- Logs debug message for troubleshooting

* refactor: use lightweight queries for cross-library move detection

Replace selectMediaFile() with newSelect() in FindRecentFilesByMBZTrackID and FindRecentFilesByProperties. These queries only need basic media file columns for hash and path comparisons, not annotations/bookmarks.

Benefits:
- Removes unnecessary LEFT JOINs with annotation and bookmark tables
- Reduces query overhead for cross-library file matching
- Follows existing pattern used by GetMissingAndMatching

The annotation/bookmark joins are user-specific (using loggedUser context) and unused in cross-library matching logic where only Equals() and IsEquivalent() checks are performed.

* test: add coverage for single-library and multi-library cross-library detection

Add test cases to verify:
1. Single-library setup correctly skips cross-library move detection
2. Multi-library setup continues to process cross-library moves

Implementation:
- New test verifies processCrossLibraryMoves returns input unchanged for single library
- Wrapped existing multi-library tests in Context with multiple libraries setup
- Ensures no regressions in multi-library matching behavior

Tests verify:
- Single-library: no database queries, input passed through unchanged
- Multi-library: cross-library matching still works correctly
- Reduces the likelihood of introducing single-library skip bugs in future

* fix: enhance cross-library detection by introducing totalLibraryCount

Signed-off-by: Deluan <deluan@navidrome.org>

---------

Signed-off-by: Deluan <deluan@navidrome.org>

scanner/phase_2_missing_tracks.go

@@ -187,6 +187,13 @@ func (p *phaseMissingTracks) processCrossLibraryMoves(in *missingTracks) (*missi
 		return nil, nil
 	}
 
+	// Skip cross-library move detection when only one library is configured
+	// since there are no other libraries to search in.
+	if p.state.totalLibraryCount == 1 {
+		log.Debug(p.ctx, "Scanner: Skipping cross-library move detection (single library)")
+		return in, nil
+	}
+
 	log.Debug(p.ctx, "Scanner: Processing cross-library moves", "pid", in.pid, "missing", len(in.missing), "lib", in.lib.Name)
 
 	for _, missing := range in.missing {