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

k-tv-backend/migrations_sqlite/20260712000002_add_chapters_and_gap_filler.sql

@@ -0,0 +1,2 @@
+ALTER TABLE library_items ADD COLUMN chapters TEXT;
+ALTER TABLE channels ADD COLUMN gap_filler TEXT;