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/local-files/src/provider.rs

@@ -47,6 +47,7 @@ fn to_media_item(id: MediaItemId, item: &LocalFileItem) -> MediaItem {
         collection_type: None,
         synced_at: None,
         role: MediaRole::default(),
+        chapters: Vec::new(),
     })
 }