add domain concepts: FillStrategy variants, Chapter, InterstitialRule, MidRollRule, gap_filler

- FillStrategy: +Alternating (round-robin series), +Weighted (fresh-first), +Marathon (always ep1, loops)
- Chapter value object on MediaItem (JSON column in library_items)
- InterstitialRule + MidRollRule on ProgrammingBlock (data model only)
- gap_filler: Option<MediaFilter> on Channel (persisted as JSON column)
- migration: 20260712000002_add_chapters_and_gap_filler.sql

crates/adapters/jellyfin/src/mapping.rs

@@ -36,5 +36,6 @@ pub(crate) fn map_jellyfin_item(item: JellyfinItem) -> Option<MediaItem> {
         collection_type: None,
         synced_at: None,
         role: MediaRole::default(),
+        chapters: Vec::new(),
     }))
 }