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/folder_db_repository.rs

@@ -8,7 +8,6 @@
 //! database triggers, so reading a folder's full path is always O(1) — no
 //! recursive CTEs or N+1 queries.
 
-use async_trait::async_trait;
 use sqlx::PgPool;
 use std::sync::Arc;
 
@@ -72,7 +71,6 @@ impl FolderDbRepository {
     }
 }
 
-#[async_trait]
 impl FolderRepository for FolderDbRepository {
     async fn create_folder(
         &self,