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/presentation/src/factory.rs

@@ -459,6 +459,7 @@ fn provider_item_to_library_item(item: domain::MediaItem, provider_id: &str) ->
         thumbnail_url: item.thumbnail_url().map(|s| s.to_string()),
         synced_at: Some(now),
         role: domain::MediaRole::default(),
+        chapters: item.chapters().to_vec(),
     })
 }