Move slot expansion logic to ExchangeService

- Add get_available_slots() and _expand_availability_to_slots() to ExchangeService
- Update routes/exchange.py to use ExchangeService.get_available_slots()
- Remove all business logic from get_available_slots endpoint
- Add AvailabilityRepository to ExchangeService dependencies
- Add Availability and BookableSlot imports to ExchangeService
- Fix import path for validate_date_in_range (use date_validation module)
- Remove unused user_repo variable and import from routes/invites.py
- Fix mypy error in ValidationError by adding proper type annotation

backend/exceptions.py

@@ -51,6 +51,16 @@ class BadRequestError(APIError):
         )
 
 
+class UnauthorizedError(APIError):
+    """Unauthorized error (401)."""
+
+    def __init__(self, message: str = "Not authenticated"):
+        super().__init__(
+            status_code=status.HTTP_401_UNAUTHORIZED,
+            message=message,
+        )
+
+
 class ServiceUnavailableError(APIError):
     """Service unavailable error (503)."""
 
@@ -59,3 +69,16 @@ class ServiceUnavailableError(APIError):
             status_code=status.HTTP_503_SERVICE_UNAVAILABLE,
             message=message,
         )
+
+
+class ValidationError(HTTPException):
+    """Validation error (422) with field-specific errors."""
+
+    def __init__(self, message: str, field_errors: dict[str, str] | None = None):
+        detail: dict[str, str | dict[str, str]] = {"message": message}
+        if field_errors:
+            detail["field_errors"] = field_errors
+        super().__init__(
+            status_code=status.HTTP_422_UNPROCESSABLE_ENTITY,
+            detail=detail,
+        )