feat(domain): add ChannelConfigSnapshot entity

k-tv-backend/domain/src/entities.rs

@@ -400,6 +400,18 @@ pub struct PlaybackRecord {
     pub generation: u32,
 }
 
+/// A point-in-time snapshot of a channel's `ScheduleConfig`.
+/// Auto-created on every config save; users can pin with a label.
+#[derive(Debug, Clone, Serialize, Deserialize)]
+pub struct ChannelConfigSnapshot {
+    pub id: Uuid,
+    pub channel_id: ChannelId,
+    pub config: ScheduleConfig,
+    pub version_num: i64,
+    pub label: Option<String>,
+    pub created_at: DateTime<Utc>,
+}
+
 impl PlaybackRecord {
     pub fn new(channel_id: ChannelId, item_id: MediaItemId, generation: u32) -> Self {
         Self {