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/domain/src/value_objects/scheduling.rs

@@ -28,6 +28,9 @@ pub enum FillStrategy {
     BestFit,
     Sequential,
     Random,
+    Alternating,
+    Weighted,
+    Marathon,
 }
 
 const DEFAULT_COOLDOWN_DAYS: u32 = 30;
@@ -99,6 +102,75 @@ pub enum MediaRole {
     Interstitial,
 }
 
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct InterstitialRule {
+    pool_filter: MediaFilter,
+    strategy: FillStrategy,
+    min_gap_secs: u32,
+}
+
+impl InterstitialRule {
+    pub fn new(pool_filter: MediaFilter, strategy: FillStrategy, min_gap_secs: u32) -> Self {
+        Self {
+            pool_filter,
+            strategy,
+            min_gap_secs,
+        }
+    }
+
+    pub fn pool_filter(&self) -> &MediaFilter {
+        &self.pool_filter
+    }
+
+    pub fn strategy(&self) -> &FillStrategy {
+        &self.strategy
+    }
+
+    pub fn min_gap_secs(&self) -> u32 {
+        self.min_gap_secs
+    }
+}
+
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct MidRollRule {
+    prefer_chapters: bool,
+    fallback_interval_mins: u32,
+    break_duration_secs: u32,
+    pool_filter: MediaFilter,
+}
+
+impl MidRollRule {
+    pub fn new(
+        prefer_chapters: bool,
+        fallback_interval_mins: u32,
+        break_duration_secs: u32,
+        pool_filter: MediaFilter,
+    ) -> Self {
+        Self {
+            prefer_chapters,
+            fallback_interval_mins,
+            break_duration_secs,
+            pool_filter,
+        }
+    }
+
+    pub fn prefer_chapters(&self) -> bool {
+        self.prefer_chapters
+    }
+
+    pub fn fallback_interval_mins(&self) -> u32 {
+        self.fallback_interval_mins
+    }
+
+    pub fn break_duration_secs(&self) -> u32 {
+        self.break_duration_secs
+    }
+
+    pub fn pool_filter(&self) -> &MediaFilter {
+        &self.pool_filter
+    }
+}
+
 #[cfg(test)]
 #[path = "tests/scheduling.rs"]
 mod tests;