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/infrastructure/services/file_system_i18n_service.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use serde_json::Value;
 use std::collections::HashMap;
 use std::path::PathBuf;
@@ -63,7 +62,6 @@ impl FileSystemI18nService {
     }
 }
 
-#[async_trait]
 impl I18nService for FileSystemI18nService {
     async fn translate(&self, key: &str, locale: Locale) -> I18nResult<String> {
         // Check if translations are cached