refactor(backend): alihkan AppConfig ke zesdex-cms JsonAppConfigRepository

Semua pemanggilan AppConfig::load() diganti dengan
JsonAppConfigRepository + AppConfigRepository trait.
Re-export model::app_config dihapus dari model/mod.rs.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>

crates/zesdex-backend/src/app/runtime/context/window.rs

@@ -5,7 +5,7 @@
 //! had their own inline version — the status bar's copy additionally
 //! displayed "?" on no match instead of falling back like the other two,
 //! an inconsistency this unifies away).
-use crate::model::app_config::AppConfig;
+use zesdex_cms::domain::app_config::AppConfig;
 use zesdex_cms::domain::settings::Settings;
 
 /// Resolve the context-window size (in tokens) for the currently
@@ -28,7 +28,7 @@ pub fn resolve(app_config: &AppConfig, settings: &Settings) -> usize {
 #[cfg(test)]
 mod tests {
     use super::*;
-    use crate::model::app_config::ModelRole;
+    use zesdex_cms::domain::app_config::ModelRole;
 
     #[test]
     fn resolves_context_window_from_matching_model_role() {