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/settings_repository.rs

@@ -1,10 +1,8 @@
 use crate::common::errors::DomainError;
-use async_trait::async_trait;
 use std::collections::HashMap;
 
 /// Repository for platform settings stored in the database.
 /// Settings are key-value pairs organized by category (e.g., "oidc", "general").
-#[async_trait]
 pub trait SettingsRepository: Send + Sync + 'static {
     /// Get a single setting value by key
     async fn get(&self, key: &str) -> Result<Option<String>, DomainError>;