fix(calendar): add IF NOT EXISTS to idx_calendar_sub_extid unique index

server/db-schema-test.js

@@ -227,7 +227,7 @@ const MIGRATIONS_SQL = {
       created_at           TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now')),
       updated_at           TEXT    NOT NULL DEFAULT (strftime('%Y-%m-%dT%H:%M:%SZ', 'now'))
     );
-    CREATE UNIQUE INDEX idx_calendar_sub_extid
+    CREATE UNIQUE INDEX IF NOT EXISTS idx_calendar_sub_extid
       ON calendar_events (subscription_id, external_calendar_id)
       WHERE subscription_id IS NOT NULL;
     CREATE INDEX IF NOT EXISTS idx_calendar_sub ON calendar_events(subscription_id);