Display sub_title (episode name) and season/episode badge on guide program cards. Extract season/episode from custom_properties with fallback parsing of onscreen_episode format (e.g. "S12 E6").
- Add season/episode SerializerMethodFields to ProgramDataSerializer
- Parse onscreen_episode string as fallback when xmltv_ns data absent
- Display sub_title with S/E badge on guide cards
- Show description for programs without sub_title (movies)
- Add formatSeasonEpisode utility with full test coverage
- Add 15 backend serializer tests and 12 frontend utility tests
Fulfills #1065
