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/domain/services/i18n_service.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use thiserror::Error;
 
 /// Error types for i18n service operations
@@ -54,7 +53,6 @@ impl Locale {
 }
 
 /// Interface for i18n service (primary port)
-#[async_trait]
 pub trait I18nService: Send + Sync + 'static {
     /// Get a translation for a key and locale
     async fn translate(&self, key: &str, locale: Locale) -> I18nResult<String>;