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/e2e/admin-invites.spec.ts

@@ -12,7 +12,7 @@ async function loginAsAdmin(page: Page) {
   await page.fill('input[type="email"]', ADMIN_EMAIL);
   await page.fill('input[type="password"]', ADMIN_PASSWORD);
   await page.click('button[type="submit"]');
-  await expect(page).toHaveURL("/audit");
+  await expect(page).toHaveURL("/admin/trades");
 }
 
 test.describe("Admin Invites Page", () => {