Fix CompoundSelect chaining: use standalone union_all()

SQLAlchemy 2.0's select().union_all() returns a CompoundSelect which
cannot chain another .union_all(). Use the standalone union_all()
function to combine all three queries.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

backend/app/services/calendar_sharing.py

@@ -7,7 +7,7 @@ import logging
 from datetime import datetime, timedelta
 
 from fastapi import HTTPException
-from sqlalchemy import delete, literal_column, select, text, update
+from sqlalchemy import delete, literal_column, select, text, union_all, update
 from sqlalchemy.ext.asyncio import AsyncSession
 
 from app.models.calendar import Calendar
@@ -45,21 +45,19 @@ async def get_accessible_event_scope(
     from app.models.event_invitation import EventInvitation
 
     result = await db.execute(
-        select(literal_column("'c'").label("kind"), Calendar.id.label("val"))
-        .where(Calendar.user_id == user_id)
-        .union_all(
+        union_all(
+            select(literal_column("'c'").label("kind"), Calendar.id.label("val"))
+            .where(Calendar.user_id == user_id),
             select(literal_column("'c'"), CalendarMember.calendar_id)
             .where(
                 CalendarMember.user_id == user_id,
                 CalendarMember.status == "accepted",
-            )
-        )
-        .union_all(
+            ),
             select(literal_column("'i'"), EventInvitation.event_id)
             .where(
                 EventInvitation.user_id == user_id,
                 EventInvitation.status != "declined",
-            )
+            ),
         )
     )
     cal_ids: list[int] = []