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/model/mod.rs

@@ -2,9 +2,6 @@
 //! plus local sub-modules (agent_def, msglog) that weren't extracted.
 
 // Module re-exports matching original `crate::model::*` paths
-pub mod app_config {
-    pub use zesdex_entities::seaorm::common::app_config::*;
-}
 pub mod store {
     pub use zesdex_entities::seaorm::common::store::*;
 }