Add calendar_events indexes and optimize dashboard queries

Migration 054: three indexes on calendar_events table:
- (calendar_id, start_datetime) for range queries
- (parent_event_id) for recurrence bulk operations
- (calendar_id, is_starred, start_datetime) for starred widget

Dashboard: replaced correlated subquery with single materialized
list fetch for user_calendar_ids in both /dashboard and /upcoming
handlers — eliminates 2 redundant subquery evaluations per request.

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

backend/alembic/versions/054_add_calendar_event_indexes.py

@@ -0,0 +1,36 @@
+"""Add performance indexes to calendar_events
+
+Revision ID: 054
+Revises: 053
+"""
+from alembic import op
+
+revision = "054"
+down_revision = "053"
+
+
+def upgrade():
+    # Covers range queries in dashboard today's events and events list
+    op.create_index(
+        "ix_calendar_events_calendar_start",
+        "calendar_events",
+        ["calendar_id", "start_datetime"],
+    )
+    # Covers bulk DELETE on recurrence edit/regeneration and sibling lookups
+    op.create_index(
+        "ix_calendar_events_parent_event_id",
+        "calendar_events",
+        ["parent_event_id"],
+    )
+    # Covers starred widget query (calendar_id + is_starred + start_datetime)
+    op.create_index(
+        "ix_calendar_events_calendar_starred_start",
+        "calendar_events",
+        ["calendar_id", "is_starred", "start_datetime"],
+    )
+
+
+def downgrade():
+    op.drop_index("ix_calendar_events_calendar_starred_start", table_name="calendar_events")
+    op.drop_index("ix_calendar_events_parent_event_id", table_name="calendar_events")
+    op.drop_index("ix_calendar_events_calendar_start", table_name="calendar_events")

backend/app/routers/dashboard.py

@@ -35,8 +35,12 @@ async def get_dashboard(
     today = client_date or date.today()
     upcoming_cutoff = today + timedelta(days=current_settings.upcoming_days)
 
-    # Subquery: calendar IDs belonging to this user (for event scoping)
-    user_calendar_ids = select(Calendar.id).where(Calendar.user_id == current_user.id)
+    # Fetch calendar IDs once as a plain list — PostgreSQL handles IN (1,2,3) more
+    # efficiently than re-evaluating a correlated subquery for each of the 3 queries below.
+    calendar_id_result = await db.execute(
+        select(Calendar.id).where(Calendar.user_id == current_user.id)
+    )
+    user_calendar_ids = [row[0] for row in calendar_id_result.all()]
 
     # Today's events (exclude parent templates — they are hidden, children are shown)
     today_start = datetime.combine(today, datetime.min.time())
@@ -173,8 +177,11 @@ async def get_upcoming(
     overdue_floor = today - timedelta(days=30)
     overdue_floor_dt = datetime.combine(overdue_floor, datetime.min.time())
 
-    # Subquery: calendar IDs belonging to this user
-    user_calendar_ids = select(Calendar.id).where(Calendar.user_id == current_user.id)
+    # Fetch calendar IDs once as a plain list (same rationale as /dashboard handler)
+    calendar_id_result = await db.execute(
+        select(Calendar.id).where(Calendar.user_id == current_user.id)
+    )
+    user_calendar_ids = [row[0] for row in calendar_id_result.all()]
 
     # Build queries — include overdue todos (up to 30 days back) and snoozed reminders
     todos_query = select(Todo).where(