Add transactional order creation API integration

Introduces saveAddOrder method in HttpService to POST orders and tickets transactionally. Refactors OrderComponent to use the new API for submitting orders and tickets in a single transaction, improving data consistency.

src/app/order/order.component.ts

@@ -161,37 +161,31 @@ export class OrderComponent {
 
 
   submitOrder(order: Bestellung, seats: Sitzplatz[], performance: Vorstellung, mode: SubmissionMode) {
-    this.httpService.addOrder(order).pipe(
-      // Order erstellen
-      switchMap(createdOrder => {
 
-        // Tickets parallel erstellen
-        const ticketObservables = seats.map(seat => {
-          const ticket = this.generateNewTicketObject(performance, seat, createdOrder);
-          return this.httpService.addTicket(ticket);
-        });
+    // Tickets anlegen
+    const tickets = seats.map(seat => {
+      return this.generateNewTicketObject(performance, seat, order);;
+    });
 
-        // Warten bis alles fertig sind
-        return forkJoin(ticketObservables).pipe(
-          tap(createdTickets => {
-            // Success Handling
-            if (mode === 'reservation') {
-              this.orderState.set({
-                status: 'reservation-success',
-                order: createdOrder
-              });
-            } else {
-              this.orderState.set({
-                status: 'purchase-success',
-                tickets: createdTickets
-              });
-            }
+    // Transaktionssicher Sitzplatzbuchung
+    this.httpService.saveAddOrder({order, tickets}).pipe(
+      tap(createdOrderAndTickets => {
+        // Success Handling
+        if (mode === 'reservation') {
+          this.orderState.set({
+            status: 'reservation-success',
+            order: createdOrderAndTickets.order
+          });
+        } else {
+          this.orderState.set({
+            status: 'purchase-success',
+            tickets: createdOrderAndTickets.tickets
+          });
+        }
 
-            this.selectedSeatsService.commit();
-            this.loadingService.hide();
-            this.showConfetti();
-          })
-        );
+        this.selectedSeatsService.commit();
+        this.loadingService.hide();
+        this.showConfetti();
       }),
       catchError(err => {
         // Error handling