Fix: Update permissions and add missing /api/exchange/slots endpoint

- Updated auth-context.tsx to use new exchange permissions
  (CREATE_EXCHANGE, VIEW_OWN_EXCHANGES, etc.) instead of old
  appointment permissions (BOOK_APPOINTMENT, etc.)

- Updated exchange/page.tsx, trades/page.tsx, admin/trades/page.tsx
  to use correct permission constants

- Updated profile/page.test.tsx mock permissions

- Updated admin/availability/page.tsx to use constants.exchange
  instead of constants.booking

- Added /api/exchange/slots endpoint to return available slots
  for a date, filtering out already booked slots

- Fixed E2E tests:
  - exchange.spec.ts: Wait for button to be enabled before clicking
  - permissions.spec.ts: Use more specific heading selector
  - price-history.spec.ts: Expect /exchange redirect for regular users

frontend/app/auth-context.tsx

@@ -17,14 +17,15 @@ export const Permission: Record<string, PermissionType> = {
   MANAGE_OWN_PROFILE: "manage_own_profile",
   MANAGE_INVITES: "manage_invites",
   VIEW_OWN_INVITES: "view_own_invites",
-  // Booking permissions (regular users)
-  BOOK_APPOINTMENT: "book_appointment",
-  VIEW_OWN_APPOINTMENTS: "view_own_appointments",
-  CANCEL_OWN_APPOINTMENT: "cancel_own_appointment",
-  // Availability/Appointments permissions (admin)
+  // Exchange permissions (regular users)
+  CREATE_EXCHANGE: "create_exchange",
+  VIEW_OWN_EXCHANGES: "view_own_exchanges",
+  CANCEL_OWN_EXCHANGE: "cancel_own_exchange",
+  // Availability/Exchange permissions (admin)
   MANAGE_AVAILABILITY: "manage_availability",
-  VIEW_ALL_APPOINTMENTS: "view_all_appointments",
-  CANCEL_ANY_APPOINTMENT: "cancel_any_appointment",
+  VIEW_ALL_EXCHANGES: "view_all_exchanges",
+  CANCEL_ANY_EXCHANGE: "cancel_any_exchange",
+  COMPLETE_EXCHANGE: "complete_exchange",
 } as const;
 
 // Use generated type from OpenAPI schema