major recurring event structure changes

backend/src/models/migrations/014_recurring_exceptions.sql

@@ -0,0 +1,5 @@
+-- Exception instances for recurring events (Google Calendar Series-Instance model)
+-- recurring_master_id: links a standalone exception instance back to its series master
+-- original_start_at: the virtual occurrence date/time this instance replaced
+ALTER TABLE events ADD COLUMN IF NOT EXISTS recurring_master_id INTEGER REFERENCES events(id) ON DELETE CASCADE;
+ALTER TABLE events ADD COLUMN IF NOT EXISTS original_start_at TIMESTAMPTZ;