Fix: Update permission names, models and constants

Permission renames:
- BOOK_APPOINTMENT -> CREATE_EXCHANGE
- VIEW_OWN_APPOINTMENTS -> VIEW_OWN_EXCHANGES
- CANCEL_OWN_APPOINTMENT -> CANCEL_OWN_EXCHANGE
- VIEW_ALL_APPOINTMENTS -> VIEW_ALL_EXCHANGES
- CANCEL_ANY_APPOINTMENT -> CANCEL_ANY_EXCHANGE
- Add COMPLETE_EXCHANGE permission

Model changes:
- Delete AppointmentStatus enum
- Delete Appointment model

Schema changes:
- Delete BookingRequest (was for old booking)
- Delete AppointmentResponse, PaginatedAppointments
- Delete BookableSlot, AvailableSlotsResponse (unused)

Constants changes:
- Remove appointmentStatuses from shared/constants.json
- Merge booking constants into exchange section
- Update shared_constants.py and validate_constants.py

shared/constants.json

@@ -8,11 +8,6 @@
     "SPENT": "spent",
     "REVOKED": "revoked"
   },
-  "appointmentStatuses": {
-    "BOOKED": "booked",
-    "CANCELLED_BY_USER": "cancelled_by_user",
-    "CANCELLED_BY_ADMIN": "cancelled_by_admin"
-  },
   "exchangeStatuses": {
     "BOOKED": "booked",
     "COMPLETED": "completed",
@@ -24,13 +19,10 @@
     "BUY": "buy",
     "SELL": "sell"
   },
-  "booking": {
+  "exchange": {
     "slotDurationMinutes": 15,
     "maxAdvanceDays": 30,
     "minAdvanceDays": 1,
-    "noteMaxLength": 144
-  },
-  "exchange": {
     "eurTradeMin": 100,
     "eurTradeMax": 3000,
     "eurTradeIncrement": 20,