fix(12-02): soft scan on launch — only scan libraries with changed file counts

SoftScanAllLibraries compares audio file count on disk vs DB track count
per library. Unchanged libraries are silently skipped (no progress bar,
no events). Only libraries where files were added or removed since the
last scan get queued for a full scan.

backend/app.go

@@ -275,12 +275,12 @@ func (yj *YellowJacketApp) OnDomReady(ctx context.Context) {
 		return
 	}
 
-	// Auto-scan all libraries on launch. Runs in a goroutine so
-	// it does not block the DOM-ready callback. Uses the same
-	// ScanAllLibraries codepath as the UI button.
+	// Soft scan: compare file counts on disk vs DB for each library.
+	// Only libraries with mismatched counts get a full scan — unchanged
+	// libraries are silently skipped (no progress bar, no UI noise).
 	go func() {
-		if err := yj.library.ScanAllLibraries(); err != nil {
-			yj.logger.Error("auto-scan failed", "err", err)
+		if err := yj.library.SoftScanAllLibraries(); err != nil {
+			yj.logger.Error("soft scan failed", "err", err)
 		}
 	}()
 }

backend/library/scan_queue.go

@@ -90,6 +90,69 @@ func (l *Library) ScanAllLibraries() error {
 	return nil
 }
 
+// SoftScanAllLibraries performs a lightweight launch-time scan.
+// For each library it compares the number of audio files on disk
+// against the track count in the database. Only libraries where the
+// counts differ (files added or removed since last scan) are queued
+// for a full scan. Libraries that are unchanged are silently skipped,
+// producing no progress-bar UI noise.
+func (l *Library) SoftScanAllLibraries() error {
+	libs, err := l.db.Queries.GetAllLibraries(l.ctx)
+	if err != nil {
+		return fmt.Errorf("could not get all libraries: %w", err)
+	}
+
+	for _, lib := range libs {
+		dbCount, countErr := l.db.Queries.CountAudioFilesByLibrary(
+			l.ctx, lib.ID,
+		)
+		if countErr != nil {
+			l.logger.Warn(
+				"soft scan: could not count DB tracks, queueing full scan",
+				"libraryID", lib.ID,
+				"libraryName", lib.Name,
+				"err", countErr,
+			)
+
+			_ = l.ScanLibrary(lib.ID)
+
+			continue
+		}
+
+		diskCount := countAudioFiles(lib.Path)
+
+		if diskCount == dbCount {
+			l.logger.Info(
+				"soft scan: library unchanged, skipping",
+				"libraryID", lib.ID,
+				"libraryName", lib.Name,
+				"tracks", dbCount,
+			)
+
+			continue
+		}
+
+		l.logger.Info(
+			"soft scan: file count mismatch, queueing scan",
+			"libraryID", lib.ID,
+			"libraryName", lib.Name,
+			"diskFiles", diskCount,
+			"dbTracks", dbCount,
+		)
+
+		if err := l.ScanLibrary(lib.ID); err != nil {
+			l.logger.Warn(
+				"soft scan: could not queue library",
+				"libraryID", lib.ID,
+				"libraryName", lib.Name,
+				"err", err,
+			)
+		}
+	}
+
+	return nil
+}
+
 // CancelCurrentScan cancels only the currently scanning library.
 // The next queued library (if any) starts automatically when the
 // current scan's goroutine completes.