fix: use POST endpoint for ticket purchase (upstream API compat)

The GET /tickets/{event_id}/user/{user_id} endpoint was a custom
addition not present in the upstream LNbits events extension. Use
the canonical POST /tickets/{event_id} with user_id in the body.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

src/modules/activities/services/TicketApiService.ts

@@ -36,6 +36,7 @@ export class TicketApiService {
 
   /**
    * Request a ticket purchase (creates a Lightning invoice).
+   * Uses POST /tickets/{event_id} with user_id in body (upstream API).
    */
   async requestTicket(
     eventId: string,
@@ -43,12 +44,14 @@ export class TicketApiService {
     accessToken: string
   ): Promise<TicketPurchaseInvoice> {
     const data = await this.request(
-      `/events/api/v1/tickets/${eventId}/user/${userId}`,
+      `/events/api/v1/tickets/${eventId}`,
       {
-        method: 'GET',
+        method: 'POST',
         headers: {
+          'Content-Type': 'application/json',
           'Authorization': `Bearer ${accessToken}`,
         },
+        body: JSON.stringify({ user_id: userId }),
       }
     )