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

Cargo.toml

@@ -26,7 +26,6 @@ futures = "0.3.32"
 async-stream = "0.3.6"
 mime_guess = "2.0.5"
 uuid = { version = "1.21.0", features = ["v4", "serde"] }
-async-trait = "0.1.89"
 thiserror = "2.0.18"
 mockall = { version = "0.14.0", optional = true }
 sqlx = { version = "0.8.6", features = ["postgres", "runtime-tokio", "tls-rustls", "chrono", "uuid", "json"] }