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/admin/trades/page.tsx

@@ -88,7 +88,7 @@ type Tab = "upcoming" | "past";
 
 export default function AdminTradesPage() {
   const { user, isLoading, isAuthorized } = useRequireAuth({
-    requiredPermission: Permission.VIEW_ALL_APPOINTMENTS,
+    requiredPermission: Permission.VIEW_ALL_EXCHANGES,
     fallbackRedirect: "/",
   });