fix: use sql.NullTime for last_played to handle NULL scan

COALESCE(last_played, '') returned empty string which can't scan into
time.Time. Removed COALESCE, use sql.NullTime instead. Format to string
only when Valid.

backend/database/sql/queries/audio_files.sql

@@ -97,7 +97,7 @@ SELECT
     af.bitrate,
     af.file_size,
     af.play_count,
-    COALESCE(af.last_played, '') AS last_played
+    af.last_played
 FROM audio_files af
 JOIN recordings r ON af.recording_id = r.id
 JOIN artist_credit ac ON r.artist_credit_id = ac.id
@@ -163,7 +163,7 @@ SELECT
     af.bitrate,
     af.file_size,
     af.play_count,
-    COALESCE(af.last_played, '') AS last_played
+    af.last_played
 FROM audio_files af
 JOIN recordings r ON af.recording_id = r.id
 JOIN artist_credit ac ON r.artist_credit_id = ac.id