feat(M003/S02): smart playlist integration — play_count and days_since_played fields

Backend:
- Added play_count and days_since_played to rule engine field whitelist
- days_since_played uses julianday() expression with COALESCE for NULL handling
- Never-played tracks (NULL last_played) match 'greater_than' but not 'less_than'
- Added PlayCount + LastPlayed to library.Track struct
- Evaluate query selects play_count and last_played from track_metadata
- Added play_count to sort field options

Frontend:
- Added play_count and days_since_played to field and numeric field lists
- Added play_count to sort options

All 49 rule engine + 15 service tests pass unchanged.

backend/library/query.go

@@ -35,6 +35,8 @@ type Track struct {
 	Channels    int64
 	Bitrate     int64
 	FileSize    int64
+	PlayCount   int64
+	LastPlayed  string
 }
 
 // genreDelimiter is the separator used by GROUP_CONCAT in the