perf: replace dyn trait objects with concrete types to eliminate vtable overhead

Remove async-trait dependency and use native Rust async fn in traits.
Replace Arc<dyn Trait> with Arc<ConcreteType> throughout the codebase
to enable monomorphization and eliminate dynamic dispatch overhead.

Key changes:
- Remove write-behind cache (no implementation existed)
- Fix should_transcode static method call
- Use ContactStorageAdapter directly instead of dyn AddressBookUseCase
- Clean up unused trait imports across services and DI

https://claude.ai/code/session_01EbAFEfyJNLRmJHmmYDX3Tt

src/application/services/i18n_application_service.rs

@@ -1,43 +1,23 @@
 use std::sync::Arc;
 
 use crate::domain::services::i18n_service::{I18nResult, I18nService, Locale};
+use crate::infrastructure::services::file_system_i18n_service::FileSystemI18nService;
 
 /// Service for i18n operations
 pub struct I18nApplicationService {
-    i18n_service: Arc<dyn I18nService>,
+    i18n_service: Arc<FileSystemI18nService>,
 }
 
 impl I18nApplicationService {
     /// Creates a dummy service for testing
     pub fn dummy() -> Self {
-        struct DummyI18nService;
-
-        #[async_trait::async_trait]
-        impl I18nService for DummyI18nService {
-            async fn translate(&self, _key: &str, _locale: Locale) -> I18nResult<String> {
-                Ok("DUMMY_TRANSLATION".to_string())
-            }
-
-            async fn load_translations(&self, _locale: Locale) -> I18nResult<()> {
-                Ok(())
-            }
-
-            async fn available_locales(&self) -> Vec<Locale> {
-                vec![Locale::English, Locale::Spanish]
-            }
-
-            async fn is_supported(&self, _locale: Locale) -> bool {
-                true
-            }
-        }
-
         Self {
-            i18n_service: Arc::new(DummyI18nService),
+            i18n_service: Arc::new(FileSystemI18nService::dummy()),
         }
     }
 
     /// Creates a new i18n application service
-    pub fn new(i18n_service: Arc<dyn I18nService>) -> Self {
+    pub fn new(i18n_service: Arc<FileSystemI18nService>) -> Self {
         Self { i18n_service }
     }