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/user_pg_repository.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use futures::future::BoxFuture;
 use sqlx::{PgPool, Row};
 use std::sync::Arc;
@@ -44,7 +43,6 @@ impl UserPgRepository {
     }
 }
 
-#[async_trait]
 impl UserRepository for UserPgRepository {
     /// Creates a new user using a transaction
     async fn create_user(&self, user: User) -> UserRepositoryResult<User> {
@@ -619,7 +617,6 @@ impl UserRepository for UserPgRepository {
 }
 
 // Storage port implementation for the application layer
-#[async_trait]
 impl UserStoragePort for UserPgRepository {
     async fn create_user(&self, user: User) -> Result<User, DomainError> {
         UserRepository::create_user(self, user)