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/services/file_content_cache.rs

@@ -198,9 +198,7 @@ pub type SharedFileContentCache = Arc<FileContentCache>;
 // ─── ContentCachePort implementation ─────────────────────────
 
 use crate::application::ports::cache_ports::ContentCachePort;
-use async_trait::async_trait;
 
-#[async_trait]
 impl ContentCachePort for FileContentCache {
     fn should_cache(&self, size: usize) -> bool {
         FileContentCache::should_cache(self, size)