refactor: simplify code - extract constants, reduce complexity

backend/src/v1/schedule_items/schemas.py

@@ -98,6 +98,12 @@ class ScheduleItemListRequest(BaseModel):
     end_at: datetime
 
 
+# Permission bit constants (matching PermissionBits in inbox_messages/schemas.py)
+_PERMISSION_VIEW = 1  # 001
+_PERMISSION_INVITE = 2  # 010
+_PERMISSION_EDIT = 4  # 100
+
+
 class ScheduleItemShareRequest(BaseModel):
     model_config: ClassVar[ConfigDict] = ConfigDict(extra="forbid")
 
@@ -109,11 +115,11 @@ class ScheduleItemShareRequest(BaseModel):
     def _permission_value(self) -> int:
         value = 0
         if self.permission_view:
-            value |= 1
+            value |= _PERMISSION_VIEW
         if self.permission_edit:
-            value |= 4
+            value |= _PERMISSION_EDIT
         if self.permission_invite:
-            value |= 2
+            value |= _PERMISSION_INVITE
         return value