feat: add shared notification, communication, and export services for bookings, courses, and events; introduce btree_gist extension and new booking atomic function

apps/web/supabase/migrations/20260416999999_enable_btree_gist.sql

@@ -0,0 +1,3 @@
+-- Enable btree_gist extension (required by booking overlap exclusion constraint)
+-- Separated into own migration to avoid "multiple commands in prepared statement" error
+CREATE EXTENSION IF NOT EXISTS btree_gist;