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/repositories/pg/settings_pg_repository.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use sqlx::PgPool;
 use std::collections::HashMap;
 use std::sync::Arc;
@@ -16,7 +15,6 @@ impl SettingsPgRepository {
     }
 }
 
-#[async_trait]
 impl SettingsRepository for SettingsPgRepository {
     async fn get(&self, key: &str) -> Result<Option<String>, DomainError> {
         let row =