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/schemas.py

@@ -6,7 +6,6 @@ from typing import Generic, TypeVar
 from pydantic import BaseModel, EmailStr, field_validator
 
 from models import InviteStatus, Permission
-from shared_constants import NOTE_MAX_LENGTH
 
 
 class UserCredentials(BaseModel):
@@ -173,51 +172,6 @@ class CopyAvailabilityRequest(BaseModel):
 # =============================================================================
 
 
-class BookableSlot(BaseModel):
-    """A bookable 15-minute slot."""
-
-    start_time: datetime
-    end_time: datetime
-
-
-class AvailableSlotsResponse(BaseModel):
-    """Response for available slots on a given date."""
-
-    date: date
-    slots: list[BookableSlot]
-
-
-class BookingRequest(BaseModel):
-    """Request to book an appointment."""
-
-    slot_start: datetime
-    note: str | None = None
-
-    @field_validator("note")
-    @classmethod
-    def validate_note_length(cls, v: str | None) -> str | None:
-        if v is not None and len(v) > NOTE_MAX_LENGTH:
-            raise ValueError(f"Note must be at most {NOTE_MAX_LENGTH} characters")
-        return v
-
-
-class AppointmentResponse(BaseModel):
-    """Response model for an appointment."""
-
-    id: int
-    user_id: int
-    user_email: str
-    slot_start: datetime
-    slot_end: datetime
-    note: str | None
-    status: str
-    created_at: datetime
-    cancelled_at: datetime | None
-
-
-PaginatedAppointments = PaginatedResponse[AppointmentResponse]
-
-
 # =============================================================================
 # Price History Schemas
 # =============================================================================