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/repositories/share_repository.rs

@@ -1,4 +1,3 @@
-use async_trait::async_trait;
 use thiserror::Error;
 
 use crate::domain::{
@@ -20,7 +19,6 @@ pub enum ShareRepositoryError {
     AlreadyExists(String),
 }
 
-#[async_trait]
 pub trait ShareRepository: Send + Sync + 'static {
     /// Save a new share or update an existing one
     async fn save(&self, share: &Share) -> Result<Share, ShareRepositoryError>;