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

backend/validate_constants.py

@@ -6,7 +6,6 @@ from pathlib import Path
 from models import (
     ROLE_ADMIN,
     ROLE_REGULAR,
-    AppointmentStatus,
     ExchangeStatus,
     InviteStatus,
     TradeDirection,
@@ -42,15 +41,6 @@ def validate_shared_constants() -> None:
             f"Invite status mismatch. Expected: {expected_invite_statuses}, Got: {got}"
         )
 
-    # Validate appointment statuses
-    expected_appointment_statuses = {s.name: s.value for s in AppointmentStatus}
-    if constants.get("appointmentStatuses") != expected_appointment_statuses:
-        got = constants.get("appointmentStatuses")
-        raise ValueError(
-            f"Appointment status mismatch. "
-            f"Expected: {expected_appointment_statuses}, Got: {got}"
-        )
-
     # Validate exchange statuses
     expected_exchange_statuses = {s.name: s.value for s in ExchangeStatus}
     if constants.get("exchangeStatuses") != expected_exchange_statuses:
@@ -69,21 +59,12 @@ def validate_shared_constants() -> None:
             f"Expected: {expected_trade_directions}, Got: {got}"
         )
 
-    # Validate booking constants exist with required fields
-    booking = constants.get("booking", {})
-    required_booking_fields = [
-        "slotDurationMinutes",
-        "maxAdvanceDays",
-        "minAdvanceDays",
-        "noteMaxLength",
-    ]
-    for field in required_booking_fields:
-        if field not in booking:
-            raise ValueError(f"Missing booking constant '{field}' in constants.json")
-
     # Validate exchange constants exist with required fields
     exchange = constants.get("exchange", {})
     required_exchange_fields = [
+        "slotDurationMinutes",
+        "maxAdvanceDays",
+        "minAdvanceDays",
         "eurTradeMin",
         "eurTradeMax",
         "eurTradeIncrement",