Initial: Rust-Backend mit Clean Architecture (domain/application/infrastructure/api)

Vier-Crate-Workspace mit:
- Domain: Account, Car, Tour, Delivery, DeliveryItem, DeliveryNote, Customer,
  Article, Warehouse, ScanState, AuditAction — alle mit serde + feature-gated
  utoipa::ToSchema.
- Application: Ports (TourRepository, DeliveryRepository, ScanRepository,
  DeliveryNoteRepository, CarRepository, AuthService) und Use Cases.
- Infrastructure: Postgres-Adapter via sqlx (PgTourRepository etc.) +
  Keycloak-AuthService mit JWKS-Cache + OIDC-Discovery.
- API: Axum 0.8, utoipa-OpenAPI + Swagger-UI, JWT-Bearer-Middleware,
  AuthenticatedUser-Extractor.

Endpoints:
- GET /me/tours/today, /tours/{id}, /accounts/{pn}, /me/cars, /health
- POST /sync/tour, /scans (bulk + idempotent via clientScanId),
  /deliveries/{id}/{hold,resume,cancel,complete,notes}, /me/cars
- PUT /tours/{id}/delivery-order, /deliveries/{id}/assigned-car, /me/cars/{id}
- PATCH /me/cars/{id}

Datenmodell:
- 6 Migrationen (accounts, tours/deliveries/items + Stammdaten,
  scan_audit mit clientScanId-UNIQUE, state_reason refactor,
  delivery_notes, cars + FKs nachziehen).
- Business-stabile Beleg-Keys (belegart_id, belegnummer) für ERP-Sync.
- Append-only scan_audit + embedded scan_state als doppelte Wahrheit.

Dev-Setup:
- docker-compose mit Postgres 17 + Keycloak 26
- Keycloak-Realm 'holzleitner' mit Public-Client (PKCE), Testfahrer
  (PN 1001) + Audience-/Personalnummer-Mapper

crates/application/src/error.rs

@@ -0,0 +1,31 @@
+use thiserror::Error;
+
+/// Fehler-Hierarchie der Application-Schicht.
+///
+/// Use Cases geben diesen Typ zurück. Die API-Schicht mappt ihn auf
+/// HTTP-Statuscodes (`NotFound` → 404, `Unauthorized` → 401, …),
+/// Persistence-Adapter mappen ihre konkreten Fehler nach
+/// `Repository(...)` hinein.
+#[derive(Debug, Error)]
+pub enum ApplicationError {
+    #[error("not found")]
+    NotFound,
+
+    #[error("unauthorized")]
+    Unauthorized,
+
+    #[error("forbidden")]
+    Forbidden,
+
+    #[error("validation: {0}")]
+    Validation(String),
+
+    #[error("repository: {0}")]
+    Repository(String),
+
+    #[error("external service: {0}")]
+    External(String),
+
+    #[error("unexpected: {0}")]
+    Unexpected(String),
+}