Phase 7: Final cleanup - Remove deprecated booking/appointment code

Deleted deprecated files:
- backend/routes/booking.py
- frontend/app/admin/appointments/, booking/, appointments/, sum/, audit/
- frontend/app/utils/appointment.ts
- frontend/e2e/booking.spec.ts, appointments.spec.ts

Updated references:
- exchange/page.tsx: Use /api/exchange/slots instead of /api/booking/slots
- useRequireAuth.ts: Redirect to /admin/trades and /exchange
- profile.tsx, invites.tsx: Update fallback redirect
- E2E tests: Update all /audit references to /admin/trades
- profile.test.tsx: Update admin redirect test

frontend/app/invites/page.tsx

@@ -21,7 +21,7 @@ type Invite = components["schemas"]["UserInviteResponse"];
 export default function InvitesPage() {
   const { user, isLoading, isAuthorized } = useRequireAuth({
     requiredPermission: Permission.VIEW_OWN_INVITES,
-    fallbackRedirect: "/audit",
+    fallbackRedirect: "/admin/trades",
   });
   const [invites, setInvites] = useState<Invite[]>([]);
   const [isLoadingInvites, setIsLoadingInvites] = useState(true);