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/bin/seed.rs

@@ -24,7 +24,7 @@ fn main() -> anyhow::Result<()> {
     // Create default app config if not present
     let config_path = store.base_dir.join("app_config.json");
     if !config_path.exists() {
-        let config = zesdex_entities::seaorm::common::app_config::AppConfig::default();
+        let config = zesdex_cms::domain::app_config::AppConfig::default();
         let content = serde_json::to_string_pretty(&config)?;
         let tmp = store.base_dir.join("app_config.json.tmp");
         std::fs::write(&tmp, content)?;