Request → [Layer₁ → Layer₂ → ... → Handler] → Response

// Each layer wraps the inner service, forming a pipeline
Router::new()
    .route("/users/{id}", get(get_user))
    .layer(CompressionLayer::new())   // Layer 3: outermost
    .layer(TimeoutLayer::new(...))    // Layer 2
    .layer(TraceLayer::new_for_http()) // Layer 1: innermost

src/
├── main.rs
├── lib.rs             # AppState, re-exports
├── error.rs           # AppError (RFC 9457)
├── extractors.rs      # Db, ValidatedJson
├── response.rs        # Created<T>, Ok<T>
└── features/
    └── users/
        ├── mod.rs
        ├── router.rs
        ├── handlers.rs
        └── models.rs  # Entity + repository
migrations/
.sqlx/                 # Commit this

async fn create(...) -> Result<Created<User>, AppError>  // 201
async fn get(...) -> Result<Ok<User>, AppError>          // 200
async fn delete(...) -> Result<NoContent, AppError>      // 204

async fn handler(Db(db): Db, Path(id): Path<Uuid>) -> Result<Ok<User>, AppError>

User::find_or_404(&db, id).await?
User::create(&db, input).await?

// ✅ External call outside tx
let external = api.call().await?;
let mut tx = db.begin().await?;
// DB only
tx.commit().await?;

PgPoolOptions::new()
    .max_connections(10)
    .acquire_timeout(Duration::from_secs(3))  // Required