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/application/ports/thumbnail_ports.rs

@@ -5,7 +5,6 @@
 //! image processing implementations.
 
 use crate::common::errors::DomainError;
-use async_trait::async_trait;
 use bytes::Bytes;
 use std::path::{Path, PathBuf};
 use std::sync::Arc;
@@ -63,7 +62,6 @@ pub struct ThumbnailStatsDto {
 /// Implementations handle the actual image processing, caching,
 /// and storage of thumbnails, while the application layer only
 /// interacts through this abstraction.
-#[async_trait]
 pub trait ThumbnailPort: Send + Sync + 'static {
     /// Check if a file is an image that can have thumbnails.
     fn is_supported_image(&self, mime_type: &str) -> bool;