feat: add index on channelId for SelectedCalendar (calcom#27403)

Add database index on the channelId column in the SelectedCalendar
table to improve lookup performance for watched calendar channels.

Author: volnei

packages/prisma/migrations/20260130000000_add_selected_calendar_channel_id_index/migration.sql

@@ -0,0 +1,2 @@
+-- CreateIndex
+CREATE INDEX CONCURRENTLY IF NOT EXISTS "SelectedCalendar_channelId_idx" ON "SelectedCalendar"("channelId");

packages/prisma/schema.prisma

@@ -1052,6 +1052,7 @@ model SelectedCalendar {
   @@index([externalId])
   @@index([eventTypeId])
   @@index([credentialId])
+  @@index([channelId])
   // Composite indices to optimize calendar-cache queries
   @@index([integration, googleChannelExpiration, error, watchAttempts, maxAttempts], name: "SelectedCalendar_watch_idx")
   @@index([integration, googleChannelExpiration, error, unwatchAttempts, maxAttempts], name: "SelectedCalendar_unwatch_idx")