wire gap_filler into schedule engine, API, MCP

crates/api-types/src/channels.rs

@@ -35,6 +35,8 @@ pub struct UpdateChannelRequest {
     pub webhook_poll_interval_secs: Option<u32>,
     pub webhook_body_template: Option<Option<String>>,
     pub webhook_headers: Option<Option<String>>,
+    #[schema(value_type = Option<Option<Object>>)]
+    pub gap_filler: Option<Option<domain::MediaFilter>>,
 }
 
 #[derive(Debug, Clone, Serialize, Deserialize, ToSchema)]
@@ -55,6 +57,8 @@ pub struct ChannelResponse {
     pub webhook_poll_interval_secs: u32,
     pub webhook_body_template: Option<String>,
     pub webhook_headers: Option<String>,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    pub gap_filler: Option<serde_json::Value>,
     pub created_at: DateTime<Utc>,
     pub updated_at: DateTime<Utc>,
 }
@@ -78,6 +82,7 @@ impl From<domain::Channel> for ChannelResponse {
             webhook_poll_interval_secs: c.webhook_poll_interval_secs(),
             webhook_body_template: c.webhook_body_template().map(|s| s.to_string()),
             webhook_headers: c.webhook_headers().map(|s| s.to_string()),
+            gap_filler: c.gap_filler().map(|f| serde_json::to_value(f).unwrap_or_default()),
             created_at: c.created_at(),
             updated_at: c.updated_at(),
         }