extract api-types crate, adopt thiserror for all errors

api-types: standalone crate with DTOs (widget, data source, layout, preset)
extracted from http-api. Shared between http-api and future SPA.

thiserror: replaced all manual Display impls with derive macros across
8 crates (config-sqlite, config-memory, tcp-server, tcp-client,
http-json, rss, media, application).

crates/adapters/http-api/src/routes/widgets.rs

@@ -6,7 +6,7 @@ use axum::{
 use domain::{ConfigRepository, EventPublisher};
 use application::ConfigService;
 use crate::AppState;
-use crate::dto::{WidgetDto, CreateWidgetDto};
+use api_types::{WidgetDto, CreateWidgetDto};
 
 type S<C, E> = State<AppState<C, E>>;