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/utils/appointment.ts

@@ -1,36 +0,0 @@
-/**
- * Appointment-related utilities.
- */
-
-export interface StatusDisplay {
-  text: string;
-  bgColor: string;
-  textColor: string;
-}
-
-/**
- * Get display information for an appointment status.
- *
- * @param status - The appointment status string
- * @param isOwnView - If true, uses "Cancelled by you" instead of "Cancelled by user"
- */
-export function getStatusDisplay(status: string, isOwnView: boolean = false): StatusDisplay {
-  switch (status) {
-    case "booked":
-      return { text: "Booked", bgColor: "rgba(34, 197, 94, 0.2)", textColor: "#4ade80" };
-    case "cancelled_by_user":
-      return {
-        text: isOwnView ? "Cancelled by you" : "Cancelled by user",
-        bgColor: "rgba(239, 68, 68, 0.2)",
-        textColor: "#f87171",
-      };
-    case "cancelled_by_admin":
-      return {
-        text: "Cancelled by admin",
-        bgColor: "rgba(239, 68, 68, 0.2)",
-        textColor: "#f87171",
-      };
-    default:
-      return { text: status, bgColor: "rgba(255,255,255,0.1)", textColor: "rgba(255,255,255,0.6)" };
-  }
-}