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/availability.spec.ts

@@ -30,7 +30,7 @@ test.describe("Availability Page - Admin Access", () => {
   });
 
   test("admin sees Availability link in nav", async ({ page }) => {
-    await page.goto("/audit");
+    await page.goto("/admin/trades");
 
     const availabilityLink = page.locator('a[href="/admin/availability"]');
     await expect(availabilityLink).toBeVisible();