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/contact_pg_repository.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use chrono::Utc;
 use serde_json::Value as JsonValue;
 use sqlx::{PgPool, Row, types::Uuid};
@@ -63,7 +62,6 @@ impl ContactPgRepository {
     }
 }
 
-#[async_trait]
 impl ContactRepository for ContactPgRepository {
     async fn create_contact(&self, contact: Contact) -> ContactRepositoryResult<Contact> {
         // Convert domain entities to persistence DTOs for JSONB serialization