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/price-history.spec.ts

@@ -61,8 +61,8 @@ test.describe("Price History - E2E", () => {
     await clearAuth(page);
     await loginUser(page, ADMIN_USER.email, ADMIN_USER.password);
 
-    // Admin should be on audit page by default
-    await expect(page).toHaveURL("/audit");
+    // Admin should be on admin trades page by default
+    await expect(page).toHaveURL("/admin/trades");
 
     // Prices nav link should be visible
     await expect(page.getByRole("link", { name: "Prices" })).toBeVisible();